Histochemical fluorescence microscopy has been used to study changes over a 15-week period in the density of catecholaminergic (CA) varicosities in medial hypothalamic nuclei of rats made hyperphagic by bilateral perifornical parasagittal knife cuts. The average daily intake of food was highest when measured two days after the knife cuts. By the 7th post-cut day, daily food intake was comparable to that measured during the 10th and 11th weeks after the cuts. The number of fluorescent varicosities in the paraventricular and periventricular hypothalamic nuclei diminished steadily after the knife cuts until they were virtually absent by the end of the fourth week. This process then reversed itself so that by the end of the 15th week the varicosities in these nuclei numbered just over 50% of presurgical values. Thus, there was an apparent lack of correspondence between increases in ingestive behavior or body weight and the nature of the CA innervation of the medial hypothalamus. This finding supports previously published work which indicates that the noradrenergic control of feeding behavior in the medial hypothalamus is mediated by a neural system that acts independently of the system responsible for the knife cut-induced hyperphagic response.