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Female Wistar rats aged 21, 18 and 16 days were injected s.c. with oestradiol benzoate (OB) at 12.00 h on Day 1 and the concentrations of LH and prolactin in the serum were measured 54 h later at 18.00 h on Day 3; the effect of a second injection of OB, given at different intervals after the first, on the concentration of gonadotrophins in the serum was also investigated. In 21-day-old rats, surges of both LH and prolactin occurred at 18.00 h on Day 3; if a second dose of OB was given within 24 h of the first, then the expected surge of LH was diminished, but if it was given on the morning of Day 3 the surge was augmented. The concentration of prolactin was, however, unaffected by the second injection. In 18-day-old rats there were no surges of gonadotrophin in response to a single OB stimulus on Day 1; however, a second dose of OB given on the morning of either Day 2 or 3 caused surges of both LH and prolactin at 18.00 h on Day 3. No such response was obtainable in 16-day-old rats.

An injection of progesterone at 12.00 h on Day 3 after a priming dose of OB at 12.00 h on Day 1 enabled surges of LH to be elicited in 18-, 16- and 14-day-old female rats but not in younger animals; surges of prolactin could be elicited to a diminishing degree in 18-, 16-, 14- and 12-day-old rats but only in younger animals if larger doses of progesterone were administered.

In further groups of young female rats an injection of progesterone was given at 12.00 h on Day 3 after the animals had been primed at 12.00 h on Day 1 with diethylstilboestrol or another synthetic oestrogen (RU2858). The results obtained paralleled data from the previous experiment in which progesterone was given after priming with the natural steroid OB.

Although the capacity for cyclicity or acyclicity in rats is normally determined early in the neonatal period, these results suggest that mechanisms concerned with the surge response to a single OB stimulus are intact and functional by 21 days of age but not in younger rats; however, it is possible to facilitate both the LH and prolactin response as early as 14 days of age.

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