The effect of somatostatin (1 microgram/kg b.w. i.v. over one minute) on regional splanchnic blood flows was studied in nine patients subjected to diagnostic angiography of the coeliac artery, the superior mesenteric artery or the aorta. Following somatostatin there was a prolongation of the circulation times and a marked constriction of the hepatic and splenic vascular beds corresponding to a 50% decrease in blood flow. Blood flow in the gastroduodenal, pancreatic, superior mesenteric and renal arteries was also decreased, but to a lesser extent. There was no effect on the lumbar arterial flow. The time for maximum filling of the portal vein was delayed and the contrasting effect in this vessel was decreased, indicating a decrease in portal flow. It is concluded that somatostatin reduces splanchnic blood flow in unanaesthetized man, while there is no decrease in skeletal muscle blood flow. It is suggested that this effect of somatostatin is due to a direct action on vascular receptor sites.