The effect of ethanol (ETOH) on glucose-stimulated insulin secretion was studied using: (1) an in vitro isolated pancreas perfusion system, and (2) an in vivo preparation utilizing unrestrained, unanesthetized rats with indwelling jugular and aortic catheters. ETOH exposure in vitro resulted in a decrease in glucose-stimulated insulin secretion from the perfused rat pancreas. Second phase secretion (min 30-60) was inhibited at low ETOH exposure (100 mg/dl) and both first (min 2-8) and second phase secretion were inhibited at higher ETOH levels (1000 mg/dl). This indicates that second phase secretion of insulin from the pancreas is more sensitive to the acute effects of ETOH than is first phase secretion. ETOH preinfusion of 4 hr in vivo resulted in an approximate 20 mg/dl decrease in plasma glucose concentrations with little or no alteration in plasma insulin levels. One hour ETOH preinfusion produced a modest 8 mg/dl fall in plasma glucose. Intravenous glucose tolerance tests following low level ETOH infusion of 4 hr resulted in an enhancement in the insulin response with no change in glucose removal. This enhancement was not observed at higher ETOH levels or after high-level, short (1 hr) ETOH preinfusion. The data suggest that stimulus-induced insulin secretion may be enhanced by an ETOH metabolite if the ETOH exposure is prolonged and at a low level. Higher ETOH concentration appears to directly block this enhancement. Due to response similarities the rat model may be of considerable value to study the effects of ETOH on stimulus-induced insulin secretion in human subjects.