Effects of age, sex and mental condition on the circadian and circannual rhythmicity of plasma prolactin in human subjects were investigated. Circannual changes were recorded 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 senile demented patients (two men and four women). Blood samples were drawn every 4 h over a 24-h period at the four sampling sessions. Circadian rhythms of the hormone were validated in all groups and at all sampling sessions except twice in elderly demented subjects. The 24-h mean levels of prolactin in plasma were approximately the same in young and elderly subjects. The circadian acrophases were most often located in the vicinity of 02.00–04.00 h. The circannual rhythmicity of the hormone showed a sex difference; the rhythm was not validated in either young or elderly men but was detected in the groups of elderly women and elderly demented patients (mainly women). The acrophases were located in May. This paper strongly suggests a sex difference in the circannual rhythmicity of plasma prolactin levels in elderly subjects.
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