Bilateral electrolytic lesions of the paraventricular nucleus of the hypothalamus (PVN) produce hyperphagia with excess weight gain. The orexigenic neuropeptide Y (NPY) system and the anorexigenic melanocortin system act in the PVN to regulate food intake, and participate in mediating the anorexic effects of leptin. We hypothesized that changes in the responsiveness of these systems may contribute to the hyperphagia observed in PVN-lesioned rats. Adult female Sprague-Dawley rats received either sham or electrolytic lesions in the PVN immediately followed by implantation of a guide cannula into the third cerebroventricle. Twenty-five days following surgery groups of sham and hyperphagic PVN-lesioned rats were injected intracerebroventricularly (i.c.v.) with either 118 pmole or 470 pmole of NPY and food intake was measured for 3 h. Food intake in response to NPY was nearly three-fold higher in PVN-lesioned rats as compared to sham rats. However, the response to 5 microg leptin i.c.v. was not different in lesioned versus sham rats. The effect of the melanocortin agonist MTII on food intake was tested in additional rats beginning either 7-14 days or 30-40 days following surgery. Doses of 0.1 nmole or 1.0 nmole of MTII were injected immediately before lights-off and food intake was measured at 2 h, 24 h and 48 h post-injection. Suppression of food intake in PVN-lesioned rats was not different from that in sham-lesioned rats. These data suggest that hyper-responsiveness to NPY may account in part for the hyperphagia observed in PVN-lesioned rats. Furthermore, based on the similarities of responses of PVN-lesioned and sham control rats to the anorexigenic agents MTII and leptin and the hypersensitivity of lesioned rats to NPY, we conclude that the PVN is not essential for NPY stimulation of food intake or for melanocortin suppression of food intake and that NPY and melanocortin receptors outside of the PVN are sufficient to produce these effects.