To investigate a possible inhibitory effect of blood drawing through an indwelling forearm vein needle on gastric acid secretion, 11 subjects were studied on four occasions each. The first session was for adapting the subject to the 3-hr collection of gastric juice. In 7 subjects the second through fourth sessions gave three conditions in balanced order: (1) an indwelling forearm vein needle and the withdrawal of 5 or 10 cc of blood every 20 min, (2) only a nonfunctional "dummy" needle implanted subcutaneously in the forearm skin, and (3) the control condition with no needle. In four additional subjects the sessions were identical except that condition (1) involved an indwelling forearm vein needle kept open by a slow infusion of saline solution and no blood was drawn. Phenol red recovery from an initial intragastric injection was measured in all. Results showed that blood drawing, but not saline infusion or venipuncture per se, inhibited gastric acid output.