Jejunoileal bypass surgery, designed to reproduce the human operation (long bypass) was contrasted with a short bypass of only a few centimeters of ileum in both genetically obese rats and their lean littermates. In comparison with the short-bypassed rats, long-bypassed rats of both genotypes lost weight and reduced food intake. When adjustments were made for group differences before surgery, no significant differences were detected in the effectiveness of the operation on the two genotypes. The reduction in food intake was accomplished by a decrease in meal size and an increase in intermeal interval. Food intake gradually returned in the long-bypassed rats toward its level in the short-bypassed rats. This increase was accomplished by an increase in meal size alone. Intermeal intervals remained prolonged. In the long-bypassed rats, compared with the short, meal duration was initially elevated and gradually increased more than did meal size, indicating a great slowing of eating rate. Water intake was unaffected, but water-to-food ratio was elevated initially and gradually declined in the long-bypassed rats. Learned food aversion, dehydration, and gastrointestinal and metabolic alterations due to malabsorption are mentioned as possible contributors to the reduction in food intake, but none of these alone can account for all the changes in meal pattern by which intake reduction is accomplished.