The synthesis and release of prolactin in rats were significantly higher at oestrus than at any other stage of the cycle, when measurements were made between 10.00 and 11.00 h (Ieiri, Akikusa & Yamamoto, 1971). Since considerable evidence indicates that stimulation of prolactin release begins on the afternoon of the day of pro-oestrus (Ieiri et al. 1971), synthesis and release were measured in the afternoon of the day of each stage of the oestrous cycle to determine the fluctuations of these parameters in the course of the cycle.
Adult female rats of the Wistar strain showing regular 4-day cycles were used. Animals were killed by decapitation between 16.00 and 17.00 h. Each isolated anterior pituitary was incubated with [14C]leucine. All the experimental conditions were the same as described previously (Ieiri et al. 1971).
The anterior pituitary functions were estimated by counting the radioactivity of [14C]leucine incorporated into prolactin