To determine the effect of somatostatin on cholecystokinin-pancreozymin (C.C.K.) release, serum-C.C.K. concentrations were measured in normal volunteers after intraduodenal olive oil, with and without a simultaneous intravenous infusion of somatostatin. After instillation of the olive oil there was a rapid rise in serum-C.C.K. (integrated response 19 682+/-5632 pg min ml-1). This rise was completely abolished by somatostatin (integrated response -373+/-330 pg min ml-1, P less than 0-005) and rebound hyper-secretion was seen after the infusion had been stopped. These findings indicate that somatostatin may be involved in regulating C.C.K. release after meals, and suggest a possible explanation for the profound steatorrhoea seen in a patient with a somatostatin-producing tumour.