Department of Hormone Research, Weizmann Institute of Science, Rehovot, Israel (Received 12 August 1975) A specific radioimmunoassay for oestradiol was used to examine at which stage of postnatal development the ovary is capable of secreting oestrogen in response to stimulation by gonadotrophins, prostaglandin E2(PGE2), or the 8-bromo-derivative of cyclic AMP (8-Br-cAMP) in vitro. Ovaries from Wistar-derived rats (6–9 days of age) were placed in incubation flasks (two ovaries per flask, at least six flasks for each experimental group) and incubated for 30 min at 37°C in 1 ml Krebs–Ringer bicarbonate buffer (KRB), pH 7·4, containing glucose (1 mg/ml) under an atmosphere of 95% O2 : 5% CO2. The medium was replaced by 1 ml fresh KRB, containing albumin (100 μg/ml), hormones, PGE2 or 8-Br-cAMP as indicated. The incubation was continued for another 4 h. The medium was extracted with 5 vol. of ether, and the ether extract was analysed for