Experiments were carried out on Sprague-Dawley rats to determine whether changes in fetal corticosterone levels during maternal diabetes were caused by the accompanying fetal hyperinsulinaemia or fetal hyperglycaemia. Diabetes was induced by injecting streptozotocin (30-45 mg/kg, i.v.) on day 2 of gestation. Fetal adrenals were removed on day 20 of gestation and cultured. Streptozotocin caused moderate (blood glucose 14-22.5 mmol/l) to severe (blood glucose greater than 25 mmol/l) diabetes. Both moderate and severe diabetes caused a decrease in fetal body weights. Relative to non-diabetic controls, maternal and fetal plasma concentrations of corticosterone were higher in the severely and lower in the moderately diabetic rats. Corticosterone production by fetal adrenal cells from control and moderately diabetic rats was comparable, but cells from the severely diabetic animals produced significantly greater amounts of corticosterone than did control cells. Neither glucose (28 mmol/l) nor insulin (1 nmol/l) exerted significant effects on [3H]thymidine uptake or corticosterone production by fetal adrenal cells from non-diabetic, moderately diabetic or severely diabetic rats. Human ACTH (0.02-20 nmol/l) caused a concentration-dependent increase in corticosterone output of comparable magnitude by cells from all three groups of animals. These data suggest that fetal growth abnormalities during diabetic pregnancy are not directly related to changes in glucocorticoid levels and that changes in glucocorticoid levels are not caused by any direct action of fetal hyperinsulinaemia or hyperglycaemia on adrenal cells.