The purpose of this study was to determine whether sensitive periods might exist in which corticosterone replacement would be more effective in normalizing puberty in adrenalectomized rats. To test this, rats were adrenalectomized at 22 days of age (when adrenalectomy will delay puberty) or at 28 days (no delay of puberty) and given either saline injections or 0.5 mg corticosterone daily beginning on the day of surgery. Three times were chosen to approximate the time of the daily trough in serum corticosterone (0800 h), the time of the peak (1600 h) and a time midway in the dark portion of the photoperiod (2300 h) when corticosterone levels were falling. Corticosterone levels produced by either the saline injection or the 0.5 mg corticosterone remained comparable even after a week of injections and were similar at the three time periods chosen. Injection of saline beginning at 22 days of age at 2300 h delayed puberty (vaginal opening and ovulation) in intact rats. Saline at other times did not. In sham-operated rats, saline at 0800 delayed puberty while other injection times did not. Saline injections in adrenalectomized rats delayed puberty more than adrenalectomy alone. Corticosterone replacement returned vaginal opening to normal. Ovulation occurred earlier in corticosterone-created rats than in saline-treated rats. Injections at all three time periods were equally effective. The data do not provide support for the hypothesis of a daily sensitive period during which corticosterone is more effective in advancing puberty in adrenalectomized rats.