Neuropeptide Y (NPY) stimulates eating in a number of species. In the studies reported here, intracerebroventricular administration of porcine NPY increased eating in mice. In the presence of food, NPY caused enhancement of water intake, whereas in the absence of food, NPY suppressed water intake. Behavioral analysis showed that NPY decreased the latency to eat, increased the time spent eating, and decreased grooming. Human NPY also increased food intake, whereas the free acid of NPY was inactive. Although some minor discrepancies in response were noted overall, NPY was as effective at stimulating food intake in genetically obese (ob/ob) mice compared with their lean littermates (ob/-), in genetically diabetic mice (db/db) and their nondiabetic heterozygote control (db/m), in streptozocin-induced diabetic mice and their controls, and in adult (8 mo old) compared with old (25 mo old) mice.