When maternal stress, containing a large anxiety component, was administered during pregnancy there was a significant decrease in 3β-hydroxysteroid dehydrogenase (3β-HSD) activity in the fetal testis from days 16 to 20 of gestation, but not at birth nor in the first week after birth. However, persistent effects were found in adult males of 90 days of age. Basal testosterone concentrations in both plasma and testes and testicular 3β-HSD activity were significantly lower whilst basal plasma progesterone concentrations were significantly higher in the stressed group. When the stressed offspring were subjected to short-term stress (one session), their plasma testosterone concentration was significantly below that of the controls. It is suggested that suppressed gonadotrophin secretion during critical periods of development alters fetal testicular function, and that raised circulating levels of stress-induced hormones such as β-endorphin may be responsible for changes in gonadotrophin secretion.