Acute administration of neuropeptide Y(NPY) into the hypothalamus and third cerebral ventricle (ICV) increases respiratory quotient, reduces energy expenditure and increases circulating, insulin, glucagon and corticosterone. Therefore, it is likely that hypothalamic NPY has acute effects on the metabolism of fuels, such as glucose. To test this hypothesis, we determined if ICV infusion of NPY influences glucose metabolism and its sensitivity to insulin in conscious, unrestrained rats, not given access to food. Glucose turnover was 4.7+/-0.3 mg/min, 45-55 min after ICV NPY was administered at 3 microg/h vs 3.7+/-0.2 (P<0.05) for ICV saline. In a time course study, glucose turnover was significantly increased 30 min, and remained elevated at 50 min after starting a similar ICV NPY infusion. In neither study was plasma glucose, insulin, glucagon or corticosterone significantly affected by ICV NPY. During an hyperinsulinaemic euglycaemic clamp, the glucose infusion rate corrected for body weight and insulin concentration, M/I was 0.22+/-0.03 for NPY vs 0.36+/-0.05 mg min(-1) kg(-1) microU(-1) ml (P<0.05) for saline. NPY treatment prevented the decline in glucose production rate but did not influence the rise in glucose disposal rate resulting from hyperinsulinaemia. It was concluded that ICV NPY rapidly stimulates glucose turnover by a mechanism that does not depend on changes in insulin, glucagon or corticosterone secretion. Furthermore, ICV NPY decreased insulin sensitivity by reducing the effect of insulin on glucose production but not on whole body glucose disposal.