Luteinizing hormone, FSH, prolactin, oestradiol and progesterone concentrations were measured by radioimmunoassay in the blood of female rats from 7 days before the first ovulation (the day of first ovulation is designated day 0) until the day after the second ovulation. Serum LH concentrations varied between 29 and 59 ng NIAMDD-rat-LH RP-1/ml from days -7 to -2. On the day preceding first ovulation (day -1) a steep increase in serum LH was found between 13.00 h and 15.00 h reaching a maximal value of 2100 ng/ml at 17.00 h. Thereafter LH levels decreased to undetectable values (less than 15 ng/ml). Serum FSH concentrations from days -7 to -2 varied between 108-154 ng NIAMDD-rat-FSH RP-1/ml; increased concentrations were found on the afternoon of day -1 and on the morning thereafter (first oestrus) with a maximal value of 1150 ng/ml at 19.00 h on day -1. On the day preceding the second ovulation (day +4) LH and FSH peaks were again found. Prolactin concentrations on the days preceding first ovulation were generally low. A small rise was found nearing first ovulation (12-7 ng/ml, day -6; 23-9 ng/ml, day -2). On day -1 a peak (118 ng/ml) was found at 15.00 h, similar to that found on day +4 in adult pro-oestrous rats. Plasma progesterone concentrations also reached a peak value (50 ng/ml) on day -1, at 17.00 h, preceded by concentrations between 3-5 ng/ml from days -6 to -2. Progesterone concentrations after the first ovulation showed a pattern characteristic of the adult cycle. Plasma oestradiol levels were un-etectable (less than 10 pg/ml) from days -6 to -2; a clearly increased oestradiol concentration was found on day -1, reaching a value of 98 pg/ml. One and two days before the second ovulation, oestradiol levels rose again. The differences and similarities between pubertal and adult pro-oestrous hormone peaks are discussed.