Diabetes was induced in female rats by streptozotocin administration prior to mating. Pregnant rats were divided into "severe diabetics" (blood glucose concentration above 300 mg/100 ml) and "mild diabetics" (blood glucose ranging from 100 to 200 mg/100 ml). When compared to control fetuses, fetuses from severely diabetic mothers showed a slight decrease of body weight on days 20.5 and 21.5. Fetal pancreatic insulin stores and plasma insulin concentrations were decreased at 19.5, 20.5 and 21.5 days. The insulin response to glucose was impaired both in vivo and in vitro. In contrast, fetuses from mildly diabetic females showed no change in body weight. Pancreatic and plasma insulin concentrations were increased at 19.5, 20.5 and 21.5 days. The response of the beta cells of term fetuses of mild diabetics to glucose stimulation was enhanced in vitro. These results are consistent with the hyperglycaemia-hyperinsulinaemia theory with regard to the fetuses from mildly diabetic rats and with an impairment of insulin biosynthesis and release in fetuses from severely diabetic females.