The pharmacokinetics of ampicillin and propylthiouracil were studied in 6 and 9 patients, respectively, before and several times up to a year after a shunt operation for extreme obesity. The drugs were given intravenously and orally making it possible to estimate the absolute bioavailability. The bioavailability of propylthiouracil (about 80%) was unchanged by the surgical procedure but the fraction of ampicillin (given as pivampicillin) absorbed decreased from a preoperative value of 109 ± 44% to 44 ± 30% 12 months after the bypass operation. Volumes of distribution transiently decreased in the postoperative period for ampicillin. Clearance was initially reduced for both ampicillin and propylthiouracil after operation but returned to normal values a year later. Half-lives of both drugs were unchanged.These results are compared with previous data on pharmacokinetics in intestinal shunt patients and a tabular review is presented. Although no general rules presently emerge from the data available, it seems prudent to closely monitor intestinal shunt patients on drug therapy by both laboratory and clinical methods.