The production of oestrogen from androstenedione by ovaries of hamsters on day 14 of pregnancy and on days 0 and 3 after birth was investigated by culturing these ovaries for 48 h in serum-free medium containing insulin and [3H]androstenedione. Both oestrone and oestradiol-17 beta were produced in culture of ovaries on day 14 of pregnancy. The production of oestrogen increased about 2-fold in culture of ovaries on days 0 and 3. Dibutyryladenosine 3':5'-cyclic monophosphate stimulated the oestrogen production by ovaries on day 14 of pregnancy and on days 0 and 3. The present results indicate that the ovaries of fetal and neonatal hamsters have an aromatase activity and that these aromatase activities are responsive to adenosine 3':5'-cyclic monophosphate.