Peripheral serum concentrations of LH, FSH and prolactin were determined by radioimmunoassay on different days after birth in the female rabbit. Serum cholesterol concentrations were also analysed by a colorimetric method. Concentrations of serum LH and FSH showed two distinct peaks occurring on day 9 (LH, 4.05 +/- 0.76 (mean +/- S.E.M.) micrograms/l; FSH, 3.62 +/- 0.67 microgram/l) and either day 35 (LH, 5.90 +/- 1.20 micrograms/l) or day 40 (FSH, 9.01 +/- 1.43 micrograms/l). Concentrations of LH and FSH were in the adult range (LH, less than 1 microgram/l; FSH, less than 0.6 micrograms/l) by day 100. Serum prolactin showed a progressive increase from day 1 (1.04 +/- 0.07 microgram/l) to day 100 (17.30 +/- 2.50 micrograms/l) with a peak on day 40. Total serum cholesterol in another series of rabbits was high from days 1 to 22 (7.53 +/- 0.64 mmol/l) and then decreased to 1.94-3.36 mmol/l by day 106. These data indicate that the two major developmental peaks in gonadotrophins may be related to ovarian function.