To establish the functional importance of a peripheral rhythm of corticosterone in the timing of the onset of vaginal opening and ovulation, rats were adrenalectomized at 22 days of age and maintained on either saline or corticosterone (B) dissolved in their drinking water. Rats that received B in their water showed a relatively constant blood concentration of B throughout the 24 h photoperiod of 5-8 �zg percent if given 80 iig B/mI water or 12-14 �zg percent if given 240 �g B/mI water. Intact 25 day old controls showed a daily rhythm of B with a trough of 6.2 ± 1.4 �g percent at 0800 h and a peak of 24.4 ± 3.5 �ig percent at 2200 h. Intact rats showed vaginal opening at 36.7 ± 0.5 days with signs of ovulation in 7 of 15 rats on that day. The remainder of the animals ovulated within two days of vaginal opening. Sham-operated rats did not differ from controls. Adrenalectomized (ADRX) rats maintained on saline showed vaginal opening at 40.1 ± 0.2 days and ovulation on that day or the subsequent two days. Puberty in rats maintained on 80 �g B/mi water was the same as in ADRX rats kept on saline. ADRX rats given 240 �zg B/mI water showed vaginal opening and ovulation at the same time as intact and sham-operated controls. It can be concluded that blood levels of B equivalent to the concentrations at the low portions of the daily rhythm were not enough to support the normal time of puberty after adrenalectomy but that intermediate levels were effective in producing normal puberty. A daily rhythm of corticosterone was not a necessary requirement for normal onset of vaginal opening and ovulation.