The effect of acute intracerebroventricular (i.c.v.) injection of neuropeptide Y (NPY) on basal and glucose- or arginine-stimulated insulinemia was studied in anesthetized and conscious rats. Basal insulinemia was not significantly increased relative to control values after NPY injection. The insulinemic response to an intravenous bolus of glucose or arginine was unaffected by prior NPY injection, glycemic profiles being identical in control and NPY-injected rats. Plasma NPY concentrations were double the corresponding control values at 20 min after i.c.v. NPY injection, but this difference was not statistically significant. Although peripheral NPY inhibits insulin secretion, these elevated plasma NPY concentrations occurred too late to explain the lack of effect of i.c.v. NPY on substrate-induced insulin secretion. Compared to control rats, marked increases in corticosteronemia were observed after i.c.v. NPY injection in conscious animals. When allowed to eat ad libitum at the end of each experiment, NPY-injected rats consumed significantly more chow in 20 min than controls. We conclude that although acute i.c.v. injection of a maximum dose of NPY had definite effects on plasma corticosterone concentrations and feeding, it favored neither the basal nor the substrate-induced insulin output.