Moderately low levels of dietary protein are associated with increased food intake and body fat. We propose that the generation of this feeding signal is dependent on the level of dietary protein relative to the protein requirement of the animal, that is, that protein-dependent feeding is maximized when the level of dietary protein is around the animal's protein requirement. One of the factors that affects an animal's protein requirement is age. We predict that young, growing animals are more responsive to a moderately low level of dietary protein than are mature animals. The feeding response to moderately low dietary protein (10% casein) was determined in young ( approximately 190 g) and more mature ( approximately 340 g) Sprague-Dawley rats for 12 days. As an index of amino acid deamination, serum urea nitrogen concentrations were determined, as was the in vitro release of neuropeptide Y (NPY) from hypothalamic tissue containing the paraventricular nucleus. Young rats were more responsive to the feeding effects of moderately low dietary protein than mature animals. In young rats, cumulative food intake was inversely correlated with serum urea nitrogen concentration. No correlation was found in mature animals. Although the amount of NPY remaining in hypothalamic tissue after incubation was significantly greater (p = 0.04) in young rats fed 10% casein as compared with rats fed the standard 20% casein diet, no dietary difference in K(+)-stimulated NPY release was observed. We hypothesize that the signal for low-protein-induced hyperphagia is a reduction in a compound whose production is coupled to the level of amino acid deamination in the brain.