Microdissected pancreatic islets of noninbred ob/ob mice were used in studies of 5-hydroxytryptamine (5-HT) and 5-hydroxytryptophan (5-HTP) effects on insulin release. The potentiating effect of 4 mM L-5-HTP on glucose-induced insulin release was inhibited by the decarboxylase inhibitors benserazide (100 microM), alpha-monofluoromethyldopa (10 or 100 microM), carbidopa (50 or 500 microM), and NSD 1015 (5 or 50 microM). Activation of L-aromatic amino acid decarboxylase by DL-m-tyrosine (4 mM) or DL-o-tyrosine (4 mM) potentiated glucose-induced insulin release, whereas L-dopa (4 mM) inhibited it. Glucose oxidation was unaffected by L-5-HTP but slightly stimulated by 5-HT. Glucose-induced efflux of 33Pi was reduced by 5-HT but not affected by 5-HTP. These results are compatible with the ideas that 5-HT inhibits glucose-induced insulin release by affecting early steps in the beta-cell stimulus-secretion coupling and that 5-HTP-potentiation of insulin release is probably mediated by the decarboxylase activity but is independent of the 5-HT formed.