Neuropeptide Y, a major hypothalamic peptide, stimulates feeding, insulin secretion and weight gain when injected intrahypothalamically. Hypothalamic NPY may be regulated by insulin availability at hypothalamic level, as its activity is apparently inhibited by intrahypothalamic insulin administration and is stimulated under insulin-deficient conditions. To determine the effects of acute physiological hyperinsulinemia, we measured regional hypothalamic NPY levels in rats during a hyperinsulinemic, euglycemic clamp. Seven male Wistar rats with implanted jugular cannulae, fasted for 24 h, were infused with insulin at 100 mU/h together with variable-rate glucose to maintain euglycemia (3.9 +/- 0.1 mmol/l), for 150 min. Controls were infused for the same period with polygeline vehicle alone (n = 8), and had blood glucose concentrations of 4.0 +/- 0.5 mmol/l. Insulin levels were 80.2 +/- 3.9 mU/l in insulin-infused rats and 15.2 +/- 1.4 mU/l in polygeline-treated controls (p < 0.001). NPY levels, measured by radioimmunoassay, were significantly higher in the arcuate nucleus/median eminence (ARC/ME) of hyperinsulinemic rats than in controls (4.8 +/- 1.2 vs 2.5 +/- 0.6 fmol/micrograms protein; p < 0.001), but were comparable with controls in 7 other hypothalamic regions. Acute physiological hyperinsulinemia therefore increases NPY levels selectively in the ARC/ME. Insulin could cause NPY accumulation in the ARC by blocking its transport to NPY-sensitive areas. This would be consistent with the suggestions that insulin inhibits hypothalamic NPY activity and also acts as a central satiety factor.