Secretion of oestrogen by the ovaries of foetal (15–19 days of gestation) and newborn rats in organ and tissue culture was not detectable by fluorometry when the ovary was taken from foetuses before folliculogenesis had occurred. In organ cultures of ovaries, the time of folliculogenesis corresponded with the normal timing of folliculogenesis in vivo. In tissue cultures the process of formation of follicles was delayed. Oestrogens were present in the medium when folliculogenesis was fully established in the cultured foetal ovaries. Secretion began spontaneously and did not depend on the addition of gonadotrophins to the medium. The addition of gonadotrophins to the culture medium did not affect the level of oestrogen secreted by the foetal and newborn rat ovaries during the period of incubation (2–3 weeks).