The jejunum of eight obese (mean weight 139 kg [306 lb 8 oz]) and four lean subjects (mean weight, 71 kg [156 lb 8 oz]) was perfused with isotonic glucose-sodium chloride solutions containing a nonabsorbable marker polyethylene glycol and, in four subjects, also folic acid, before and after a five-day total caloric fast. Absorption of glucose, water, sodium, chloride, and folic acid and secretion of urea and potassium were determined. The absorptive capacity of the substances measured was identical in normal-weight and obese groups. After the five-day fast, the absorption of glucose, water, sodium, chloride, and folic acid and the secretion of urea declined significantly. Starvation did not alter the jejunal histological findings in three subjects in whom small bowel biopsies were done, but activities of alkaline phosphatase, sucrase, and maltase in the small bowel mucosa decreased.