Precocious ovulation was induced in 26-day-old rats by means of 25 IU pregnant mare’s serum gonadotrophin (PMS). Rats were given injections at either 09.00 or 16.00 in order to determine whether the course of pubertal changes in corticosterone (B), progesterone (P) or time of ovulation would be dependent upon the time of injection. Blood samples were collected by decapitation at 4-h intervals on the day of expected ovulation (day 28). In saline-treated controls at 28 days of age there was a demonstrable daily variation in both serum B and P with maximum values at 16.00 and minimum values at 08.00. In rats given PMS at 09.00, there was a peak of P and B at 16.00 on day 28 but the values were higher than in controls. Ovulation took place at 24.00 on day 28–29. In rats given PMS at 16.00 the pattern of P and B was different, with continual high values from 16.00 to 04.00 at 28 days of age; ovulation took place later in this group (between 02.00–08.00 on day 28). It can be concluded that there is a daily rhythm of P a few days before puberty and that the induction of precocious ovulation with PMS is associated with a different pattern of B and P and a different time of ovulation depending upon when PMS is adminis