Studied feeding behavior in 4 experiments with a total of 86 Zucker rats. Ad-lib food and water intakes were significantly greater for the genetically obese rats (fatties) than for their nonobese littermates. The ratio of water intake per gram of food intake was not different for the 2 groups. The ability to regulate caloric intake was then tested in 4 groups of rats: genetically obese, ventromedial hypothalamic lesioned (VMH) obese, sham operated, and normal controls. In response to caloric dilution and quinine adulteration the genetically obese Ss behaved more like normal Ss than like VMH-lesioned Ss. Sensitivity to quinine increased with age in the Zucker fatty. The fact that Ss with genetic obesity and Ss with hypothalamic obesity displayed different behaviors suggests that obesity is not merely a unitary disorder. (20 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)