Aged (12- to 14-month-old) C57BL oestrous mice exhibited significantly lower (P less than 0-001) ovarian delta 5-3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) concentration, specific activity and total content than young (3-month-old) oestrous mice, suggesting a decrease in the potential of the older animals to produce ovarian delta 4-3-oxosteroids. Mice in both age groups maintaining pregnancy to 10 or 18 days post coitum (p.c.) had similar values for activity of ovarian 3 beta-HSD. In the aged females in which foetal resorption had occurred, the majority of foetuses had been resorbed by 10 days p.c. However, ovarian 3 beta-HSD activity in these animals was not significantly different from that of young or aged mice maintaining pregnancy. By day 18, however, ovarian dehydrogenase activity in aged females failing to maintain pregnancy had decreased significantly. It is suggested that foetal death in aged mice is not the result of a deficiency in ovarian 3 beta-HSD, but rather may initiate luteal regression and consequently a decline in 3 beta-HSD.