Neuropeptides have been implicated in the short-term regulation of food intake and the long-term control of body weight. Previous studies have shown that central administration of neuropeptide Y (NPY), the most abundant of these peptides in the brain, produces robust increases of food intake. We now report that NPY, at doses that stimulate food intake when administered intraventricularly, also causes the formation of robust conditioned flavor aversions when given via the same cannula and at the same dose. This apparently paradoxical effect may be indicative of different populations of central NPY receptors having dissimilar effects on ingestive behaviors. The results also suggest that the use of conditioned aversions to investigate drug-induced malaise may not be appropriate when applied to ingestive behaviors.