The effect of L-arginine on fetal insulin release has been investigated in the chronically catheterized sheep fetus and the findings have been compared with the beta cell response to glucose. An infusion of arginine for 5 min (200 mg as a solution of 11.5 mmol/l) stimulated a rapid release of insulin in all the fetuses from 119 to 142 days of gestational age. The maximum insulin concentration occurred at the end of infusion; the mean insulin increase between 0 and 5 min was 18.0 +/- 2.0 mocrounits/ml (n = 20). The response to infusion of glucose for 5 min (1 g as a solution of 2.8 mol/l) was more variable and slower than that to arginine with the peak concentration of insulin occurring 10 min after the end of infusion. The mean increase in the concentration of insulin between 0 and 15 min in response to glucose was 42.5 +/- microunits/ml (n = 11). The response to arginine but not to glucose was related to the gestatinal age of the fetus. The increment in insulin in response to glucose was dependent upon the initial concentration of fetal plasma glucose. The observations are discussed in relation to the regulation of insulin release in utero and the mechanisms of insulin secretion.