The amount of oestrogen produced by immature mouse ovaries cultured with purified gonadotrophins was measured by means of a radioimmunoassay. Small quantities were released in response to FSH or LH alone, but when both were provided production was greatly enhanced after an initial lag phase of 2–3 days. The lowest concentrations required to produce this synergistic effect were between 0·02 and 0·13 i.u. FSH/ml and between 0·01 and 0·1 i.u. LH/ml. After prolonged exposure to 0·4 i.u. FSH/ml plus 1·0 i.u. LH/ml, oestrogen output continued to rise until, on the ninth day of culture, it reached 2·6–7·3 ng/ ovary/day. Pre-treatment with 0·4 i.u. FSH/ml for 3 days enhanced the subsequent response to combined gonadotrophins but the simultaneous presence of FSH and LH was essential for inducing the delayed synergistic effect. Although both gonadotrophins also stimulated follicle growth there was no evidence suggesting any simple correlation between that response and oestrogen synthesis. Maximal oestrogen production coincided with very low follicular mitotic activity.