N-[(trans-4-isopropylcyclohexyl)-carbonyl]-D-phenylalanine (A-4166) has a structure which differs from those of other known blood glucose-lowering agents including sulfonylureas. It has been shown that oral administration of A-4166 exerts blood glucose-lowering effects in animal in vivo studies. In the present study, we investigated the effects of A-4166 on insulin and glucagon secretion at several glucose concentrations using isolated perfused rat pancreas preparations. Both 3.0 and 30 mumol/l A-4166 significantly stimulated insulin secretion as compared with basal levels at glucose concentrations of 8.0 and 11.0 mmol (p < 0.01 and p < 0.05, respectively). In contrast, glucagon secretion was not affected by administration of A-4166 up to 30 mumol/l at these glucose concentrations. At a glucose concentration of 5.6 mmol/l, neither 0.3 nor 3.0 mumol/l A-4166 produced significant changes in insulin and glucagon secretion. However, A-4166 at 30 mumol/l significantly stimulated insulin secretion and inhibited glucagon secretion as compared with basal levels (p < 0.01 and p < 0.01, respectively). We conclude that A-4166 at 3.0 and 30 mumol/l directly stimulates insulin secretion but has little effect on glucagon secretion in isolated perfused rat pancreas at glucose concentrations of 8.0 and 11.0 mmol/l. these results, taken together with previously published data, suggest that oral administration of A-4166 could be a useful strategy for stimulating endogenous insulin secretion in non-insulin-dependent diabetic patients.