As streptozocin has a toxic effect on gastrin producing cells in some patients with gastrinomas, the action of the drug upon normal gastrin release was evaluated in patients with carcinoid tumours (n=6) and malignant insulinomas (n=2). No acute effects were recorded in 22 instances where gastrin levels were followed during the first 24 hours after infusion of streptozocin. When gastrin levels were compared throughout a course of repeated infusions during months a significant increase was noted. Concentrations were doubled after 6 g streptozocin given during a four months period, and tripled after 10 g in nine months period. One patients developed bleeding duodenal ulcer after a total dose of 6 g. It is concluded that streptozocin does not damage normal G cells, but by some action seems to stimulate gastrin relase. Peptic ulceration may be an important side effect during a long term treatment.