In order to study the role of the adrenal cortex in the onset of puberty, female Sprague-Dawley rats were obtained at weaning age and were placed on a 14 h light: 10 h dark cycle. Precocious puberty was produced by means of pregnant mare’s serum gonadotrophin (PMS) given at 22 or 26 days of age. Blood was collected daily at noon or at 8 p. m. by cardiac puncture from alternate groups of rats and assayed for corticosterone (B) by means of a fluorometric procedure. 5 IU PMS at 22 days of age elicited vaginal opening in 20/38 rats on day 24. Evidence of ovulation in these rats was found the following day. A rise in serum B occurred on the morning of day 24 (23.5 ± 1.7 µg% vs. 6.7 ± 0.4 µg% in saline controls) in those rats that ovulated and on the evening of day 23 in those that did not ovulate (26.6 ± 4.8 µg% vs. 11.0 ± 0.8 µg% in saline controls). Facilitation of ovulation with 0.5 mg progesterone at 9 a.m. on day 24 resulted in a depression of B on days 24 and 25, but ovulation occurred in all of the treated rats. The rise in serum B after 25 IU PMS was greater in rats treated on day 26 than in rats treated on day 22. Ovariectomy blocked the response to 25 IU PMS on day 22 but did not prevent the rise in B in animals on day 26. The response to 5 IU PMS was similar in intact and ovariectomized rats at both ages. 25 IU PMS at 4 p.m. on day 26 failed to elevate B to a level comparable to the effects of 25 IU PMS at 9 a.m. on that day. Superovulation, produced by the injection of 25 IU HCG at 9 a.m. on day 29, after 25 IU PMS at 4 p.m. on day 26, suppressed the diurnal rhythm and produced serum B values intermediate between the peak and trough values of other groups. Some recovery was seen by day 33. It is suggested that the change in response to PMS with age may be due to the need for priming levels of adrenocortical hormones to produce ovulation in younger rats treated