Administration of neuropeptide Y (NPY) into hypothalamic areas or into the cerebral ventricles induces marked increases in food consumption in satiated rats. Since monoamines have been suggested to be involved in NPY-induced feeding, we investigated the effects of NPY and [D-Trp32]NPY, a putative NPY antagonist, on extracellular levels of norepinephrine (NE), dopamine (DA), serotonin (5-HT), 3,4-dihydroxyphenyl acetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindole-3-acetic acid (5-HIAA) in the hypothalamus, including the paraventricular hypothalamic nuclei (PVN), during feeding behavior. Intracerebroventricular (i.c.v.) injections of NPY (20 microg) significantly increased extracellular NE (1.5-fold), DA (2.5-fold), DOPAC (2-fold) and HVA (3-fold), and did not change 5-HT or 5-HIAA levels. This dose of NPY significantly increased food intake over a 2 h period. The putative NPY antagonist [D-Trp32]NPY (40 microg, i.c.v.) produced similar neurochemical changes to NPY: it increased dialysate levels of NE (1.7-fold), DA (2.5-fold), DOPAC (1.6-fold) and HVA (2.2-fold) and did not change 5-HT or 5-HIAA levels. [D-Trp32]NPY also produced a significant increase in food intake. I.c.v. administration of [D-Trp32]NPY 5 min before NPY did not significantly change the increase in NE, DA, HVA and DOPAC induced by NPY. In these animals, food consumption was also significantly increased. These data indicate that NPY-induced feeding is associated with activation of the hypothalamic monoaminergic system and that [D-Trp32]NPY, at the dose given, acts as an agonist and not as an antagonist at NPY receptors in vivo.