The suppression of food intake after a period of forced overfeeding is potent and long lasting, yet little is known of the underlying mechanisms for this regulatory response. Rats were overfed via a surgically implanted gastrostomy tube. During overfeeding, plasma insulin and corticotropin-releasing hormone (CRH) mRNA in the paraventricular nucleus of the hypothalamus were elevated compared with controls and with overfed rats allowed 3 days to recover from the overfeeding regimen such that body weight returned to the level of controls. In contrast, rats that were not overfed but were pair-fed to the low spontaneous food intake of previously overfed rats lost weight and had significantly reduced plasma insulin and elevated mRNA for neuropeptide Y (NPY) in the arcuate nucleus of the hypothalamus. The results indicate that overfeeding produces an activation of hypothalamic CRH system that may contribute to the hypophagia that accompanies involuntary overfeeding. Furthermore, the hypothalamic NPY response to food restriction is not tied to low food intake per se, but rather to negative energy balance.