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Circannual changes of immunoreactive LH and FSH were documented on a circadian basis in January, March, June and October in four groups of subjects: seven young men, six elderly men, six elderly women and six men and women suffering from senile dementia. The sampling was serially dependent only for the young men and the core subgroups of elderly men and elderly women. A circadian rhythm for FSH was not detected in any group of subjects during any of the sampling sessions, whereas a circadian rhythm for LH was detected twice (June and October) in young men, once (October) in elderly demented patients, and not at all in the groups of elderly men and women. Both 24-h and yearly mean levels of gonadotrophins were higher in elderly subjects (two-to 25-fold according to the hormone, sex and season) than in young men. Circannual rhythms of plasma LH with large amplitudes were validated by the cosinor method, with an acrophase located in April or May. A circannual rhythm of plasma FSH was validated only in young men, with an acrophase in October. The persistence of a circannual rhythm of plasma LH with large amplitude in elderly subjects, associated with high mean levels of the hormone, especially in elderly women, suggests that this bioperiodicity of the pituitary gland is independent of gonadal function.

Department of Physiology and Biophysics, Colorado State University, Fort Collins, Colorado 80523, U.S.A.

(Received 13 September 1977)

Mammalian pineal gland activity is controlled by environmental lighting schedules. Light exerts its influence via a neuronal pathway originating in the retina (Moore & Klein, 1974) and as a consequence of this photoperiodic control, the concentration of melatonin in the plasma is raised during periods of darkness and depressed during periods of light (Rollag & Niswender, 1976). The response of the pineal gland to photostimulation is surprisingly rapid. Within 5 min of a darkness to light transition, there is a precipitous decline in pineal N-acetyltransferase activity in the rat (Deguchi & Axelrod, 1972; Klein & Weller, 1972). In sheep, peripheral concentrations of melatonin decline within 5–10 min of a darkness to light transition (Rollag, O'Callaghan & Niswender, 1978). A circadian rhythm of blood flow to the pineal gland analogous to the rhythm of melatonin

The occurrence of circadian variations in the concentration of prolactin in the plasma of 6- to 9-month-old male rats has been assessed in animals exposed to light for 14 h/day (lights on 06.00–20.00 h). Blood samples were obtained after decapitation, or from individual rats at regular intervals via a permanent cannula. Care was taken to limit stress during sampling. The concentration of prolactin in the plasma was significantly lower between 07.00 and 15.00 h than at other times. Between 15.00 and 20.00 h (during the light period), the concentration of prolactin was significantly higher in comparison with the preceding period, or with the remainder of the 24 h period. During the night, the concentration fluctuated, probably because of episodic releases of the hormone. The possible physiological significance of a circadian rhythm in the plasma concentration of prolactin and the implications for endocrine experimentation are discussed briefly.

Department of Animal Science and Production, University of Western Australia, Nedlands, Western Australia 6009, Australia

(Received 13 March 1978)

The activity of the adrenal gland is believed to be governed by the secretion of corticotrophin (ACTH) in a positive stimulus/negative feedback equilibrium. There is increasing evidence that in man, the secretion of corticosteroids never actually reaches a steady-state condition and that the circadian rhythm displayed by these hormones in the circulation is therefore the result of a number of secretory episodes over a 24 h period (Hellman, Nakada, Curti, Weitzman, Kream, Roffwarg, Ellman, Fukushima & Gallagher, 1970; Weitzman, Fukushima, Nogeire, Roffwarg, Gallagher & Hellman, 1971). Data presented by McNatty, Cashmore & Young (1972) also raise the possibility that a similar pattern of hormone release may exist in the sheep. However, McNatty et al. (1972) collected samples relatively infrequently and it is hard to define peaks in cortisol concentration. With more

Radioimmunoassays were used for the measurement of several androgens in canine plasma and in the liquid of the vas deferens. Large variations in the plasma concentrations of androstenedione, testosterone and 5α-dihydrotestosterone occurred during a period of 24 h, but there was no evidence of a circadian rhythm. The ratios of the androgen concentration in the liquid of the vas deferens compared with that in the peripheral plasma were: androstenedione, 4·6; testosterone, 1·9; 5α-dihydrotestosterone, 13·6; 5α-androstane-3α,17β-diol, 17·0; 5α-androstane-3β,17β-diol, 22·4. These high levels of androgens in the liquid of the vas deferens could play a role in the development of prostatic hypertrophy.

Ovariectomized rats in which <7% of the suprachiasmatic nuclei had been spared by bilateral radiofrequency lesions were distinguishable from those with >40% of the nuclei by their consistent failure to show the oestrogen-induced daily surge of LH, either with or without pharmacological manipulations of serotonin (5-HT), and also by their loss of the normal rhythmicity of drinking. Minor damage to structures adjacent to the suprachiasmatic nuclei was similar in both groups. The identical facility with which electrical stimulation of the preoptic area induced LH release in the two groups of animals suggested that they were not characterized by different degrees of damage to the preopticotuberal pathway. These results are considered in relation to evidence indicating that the suprachiasmatic nuclei represent the densest concentration of 5-HT terminals in the forebrain and also the site of a mechanism involved in the generation of circadian rhythms.

It is well-known that adrenocortical secretory activity shows a characteristic oscillating course with a 24-h period. This is promoted by a corresponding circadian rhythm of corticotrophin (ACTH) secretion, normally related to the alteration of sleep with the waking state. In the urine, this phenomenon is expressed by a urinary steroid flow which rises during the morning hours reaching a maximum between 08.00 and 12.00 h and subsequently drops to its lowest level at about 24.00 h. It is thus possible to assess the effects on the adrenal cortex of a strictly physiological corticotrophic stimulus such as that to which the adrenal cortex is subjected during the early morning hours (Ceresa, Angeli, Boccuzzi et al. 1969, 1970). In fact, any qualitative change induced by endogenous ACTH in adrenal steroid secretory activity may be detected at the urinary level by variations of the metabolic pattern.

The present investigation was done in 16

It is now well established that several hormones are secreted in an episodic or pulsatile fashion (McNatty, Cashmore & Young, 1972; Murray & Corker, 1973). When plasma concentrations of certain hormones are examined, peaks occur at intervals related to different features of the 24-h cycle, circadian rhythms (Retiene, Zimmerman, Schindler, Neuenschwander & Lipscomb, 1968). Circadian and episodic patterns in thyroid function have been claimed by certain workers (Bakke & Lawrence, 1965; Blum, Greenspan & Magnum, 1968) and refuted by others (Schatz & Volpe, 1959; Odell, Wilber & Utiger, 1967). A partial explanation of these conflicting results probably lies in functional differences in the level of the hypothalamo-pituitary-thyroidal-peripheral target tissue axis under investigation by different techniques.

Five Jersey bull calves aged between 2 and 80 days were investigated on a total of nine occasions. Five experiments were conducted under natural lighting conditions (daylight from 04.30 to 19.30 h). Calves in the

Recent studies of the circadian rhythm of steroidal and gonadotrophic hormones (Hellman, Nakada, Curti, Weitzman, Kream, Roffwarg, Ellman, Fukushima & Gallagher, 1970; Katongole, Naftolin & Short, 1971; Sederberg, Binder & Kehlet, 1971; Naftolin, Yen & Tsai, 1972) have shown that it is far from being the smooth diurnal variation usually reported. Instead rapid fluctuations in the levels of the hormones occur throughout the day. The present study was undertaken to confirm the findings of Naftolin et al. (1972) regarding luteinizing hormone (LH) and to attempt to correlate the plasma levels of LH with changes in plasma testosterone. Blood samples (5 ml) were obtained at 10-min intervals over an 8-h period, between 09.00 and 17.00 h, from three normal men. The men (aged 25–30 years) were recumbent throughout the experiment, except when urinating, and were awake and allowed to read or converse. Frequent sampling was facilitated by the use of an