This study was undertaken to determine the influence of somatostatin on the growth of gastric, duodenal, and pancreatic tissue in rats placed on liquid diet. In the first part of the study animals were fed an elemental liquid diet for 10 days and then killed, and the growth of the oxyntic gland area of the stomach, 2 cm segments of duodenum and pancreatic tissue was determined. Feeding an elemental diet caused a decrease in organ weight, nucleic acid contents and serum gastrin level. Subsequent addition of pentagastrin prevented this reduction in organ weight and RNA and DNA contents while somatostatin failed to affect the decrease in growth parameters or serum gastrin level in tests with or without addition of pentagastrin. In a second part of the study, sham operated and antrectomised rats were used. Antrectomy caused a significant decrease in serum gastrin concentration and resulted in a significant reduction in the weight and RNA and DNA contents of the tissue tested. As in liquid diet, subsequent administration of pentagastrin prevented the reduction in the growth parameters both in tests with and without somatostatin. These results suggest that somatostatin inhibits the growth of the gastroduodenal mucosa by a mechanism involving, at least in part, the suppression of gastrin release.