SUMMARY The concentration of progesterone and oestradiol-17β in peripheral plasma was determined by radioimmunoassay on each day of pregnancy in the mouse. The progesterone concentration, low on days 1 and 2, correlated with the histological development of the corpora lutea. Progesterone levels during the first half of pregnancy rose to a peak on day 6 with a significant decrease (P < 0·02) on day 7. Peripheral progesterone concentration during the second half of pregnancy rose on days 12–13 with a peak on days 14–17, and declined after day 17, 3 days before the onset of parturition. Oestradiol-17β concentrations were high on day 1, declined precipitously to low levels on days 2 and 3 recovering slightly on day 4. The levels then fell and remained low from day 5 to days 15–16, when there was a significant increase in oestradiol-17β concentration which continued until day 20.

Follicle-stimulating hormone (FSH), luteinizing hormone (LH) and prolactin concentrations were determined by radioimmunoassay (NIAMD-Rat RIA) of sera from intact hamsters, which were decapitated at selected times during the 4-day estrous cycle. Serum FSH concentrations on Day 1 of the cycle (day of ovulation) were 2-to 6-fold greater than the levels on Day 2 and 5 to 12-fold greater than the mean, "basal," level of FSH (53 ± 2 ng/ml; 80 hamsters) found during Day 3 and until 1200 hr, Day 4. There was a biphasic serum FSH pattern on Day 4 (proestrus) which was demarcated by the initiation of the preovulatory surge at 1400 hr, the peak level at 1600 hr, the decay of the surge by 2100 hr, and the subsequent elevation to the high levels present on Day 1. In marked contrast to these changes in serum levels of FSH, LH levels were virtually constant throughout the estrous cycle, except for the preovulatory surge between 1300 hr and 1900 hr on Day 4 with a peak level at 1600 hr. Prolactin concentrations varied at most by one-half to 2-fold from each other during diestrus. The preovulatory increase in serum prolactin on proestrus was at a maximum level at 1500 hr, then declined by 2000 hr. (Endocrinology 94: 1295(Endocrinology 94: , 1974 LOOD levels of luteinizing hormone (LH), determined by ovine:ovine radioimmunoassay, in the cyclic hamster have been reported by several groups of investigators (1-3). However, peripheral levels of follicle-stimulating hormone (FSH) and prolactin during the estrous cycle of the hamster have not been measured.Previous studies from this laboratory have centered on follicular (4) and luteal (5) development during the cycle and preovulatory changes in the ovary and pituitary during proestrus (6,7). To further delineate the reproductive processes occurring during the estrous cycle of the hamster, the objective of the present investigation was to determine the serum FSH, LH and prolactin during the cycle.

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