The intrapancreatic localization and the effects on basal and stimulated insulin secretion of neuropeptide Y (NPY) were investigated in the mouse. Immunocytochemistry showed NPY to be confined to intrapancreatic nerve fibers mainly associated with blood vessels. Fine varicose NPY fibers were also detected in the exocrine parenchyma and occasionally also within the islets. Double-staining experiments with the use of antisera for both NPY and tyrosine hydroxylase (TH) indicated that most of the NPY fibers were nonadrenergic in nature. Only a population of the NPY fibers occurring around blood vessels showed TH immunoreactivity. Under in vivo conditions, NPY was found to elevate plasma insulin levels slightly when injected intravenously at the high dose level of 8.5 nmol/kg. At lower dose levels, NPY did not affect basal plasma insulin levels, but instead inhibited glucose-induced insulin secretion. Thus, the glucose-induced increment in plasma insulin levels, which was 120 +/- 7 microU/ml in controls, was reduced to 87 +/- 5 microU/ml by NPY at 4.25 nmol/kg (p less than 0.01) and to 98 +/- 6 microU/ml by NPY at 1.06 nmol/kg (p less than 0.05). In contrast, the insulin secretory response to the cholinergic agonist carbachol was not affected by NPY. We conclude that NPY nerve fibers occur in the mouse pancreas and that most of these NPY nerve fibers are nonadrenergic. Furthermore, in the mouse, NPY enhances basal plasma insulin levels at high dose levels and inhibits glucose-induced, but not cholinergically induced insulin secretion at lower dose levels under in vivo conditions.