Diurnal variations in concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), thyroid-stimulating hormone (TSH), corticosterone, estradiol-17 beta and estrone in serum were investigated in 27- to 35-day-old female rats maintained on a 14-hour light and 10-hour dark schedule. Groups of 6-8 rats were decapitated at 3-hour intervals during a 24-hour period. Hormones were measured by radioimmunoassay. LH levels were highly variable over a 24-hour period in rats younger than 32-33 days of age, while in 34- to 35-day-old animals the levels were less variable and a moderate LH peak was seen at 24.00 h. A distinct FSH diurnal rhythm with the peak at 15.00 h was demonstrated in rats older than 32 days of age. The 27- to 28-day-old rat had a diurnal TSH rhythm with the peak at 12.00 h, but older rats had a rhythm with two peaks at 24.00 and 9.00 h. The diurnal corticosterone rhythm analogous to the adult's was established at 32 days of age, although a significant variation having a peak during the dark period appeared earlier. Similarly, a distinct estradiol rhythm appeared at 29 days of age with the peak at 12.00 h, followed by a change in the peak phase afterwards. Serum estrone levels showed a manifest variation in the 27- to 28-day-old rat, but no variation was observed in the older rats. These results not only confirm the previous reports on the existence of pulsatile LH secretion, and corticosterone and estradiol diurnal rhythms, but also demonstrate the existence of diurnal rhythms in FSH, TSH and estrone levels in the immature female rat. Furthermore, it is possible that the rhythm once developed may change in phase, or even disappear as puberty approaches.