Insulin concentrations were measured in 20 chronically instrumented fetuses of 10 sheep with alloxan-induced chronic hyperglycemia and 10 control sheep to examine if the hyperglycemia resulted in high fetal insulin concentration. Additionally, in six neonatal lambs of three chronically hyperglycemic ewes and three control sheep, insulin concentrations were measured after intravenous glucose injection. After a 2-month period of significant maternal hyperglycemia, no relationship between concentrations of fetal glucose and insulin could be detected. The mean fetal glucose concentration was 3.5 +/- 0.5 mmol/L in the hyperglycemic group and 0.6 +/- 0.1 mmol/L in the control group. Mean fetal insulin levels were 12.5 +/- 1.4 and 10.7 +/- 1.3 microU/ml, respectively. The neonatal lambs of the hyperglycemic and control ewes showed comparable concentrations of glucose and insulin after infusion of glucose. It is presumed that persistent high glucose levels depress the insulin secreting capacity of the fetal pancreas.