This study deals with the influence insulin exerts upon pancreatic somatostatin release. In different incubation systems, isolated rat islets were subjected to addition of either exogenous rat insulin or of anti-insulin serum (AIS). No change in somatostatin release was caused by insulin (25 U/l) during perifusion of freshly isolated islets at 8.3 mmol/l glucose, whereas AIS showed an inhibitory effect. During a 42 hr culture period, somatostatin content of culture medium remained low in the presence of insulin but was elevated when AIS has been added. Therefore, further experiments were carried out after a 2-day period of precultivation. Precultivated islets responded to exogenous insulin with a decrease in somatostatin release in static incubations and in perifusion experiments. It is suggested that the lack of insulin effect in freshly isolated islets is due to alterations in receptors which are restored during islet culture. The inhibition of somatostatin release from freshly isolated islets observed during perifusion with AIS may be caused by anti-insulin receptor antibodies probably present in the anti-insulin serum.