Serum gastrin concentrations were measured in insulin-dependent diabetics and in patients who had undergone total duodenopancreatectomy and gastrectomy. Administration of somatostatin markedly inhibited gastrin release in the diabetic patients with an intact gastrointestinal tract. This inhibitory effect of somatostatin, however, is not detectable in the group of operated patients, who display basically low plasma gastrin levels. A marked increase in the plasma gastrin level after withdrawal of somatostatin was observed in patients with a normal gastrointestinal tract. This rebound effect was only slight in the operated patients.