The action of watery rat gut extracts on glucose-induced insulin release in anaesthetized rats was examined before and after removal of GIP by immunoadsorption. Infusions of GIP-containing rat gut extracts nearly doubled the insulin release induced by intravenous glucose (1 g X kg -1 X h -1). Peak insulin secretion was 98 +/- 11 mU/l (mean +/- SEM) after intravenous glucose and increased to 178 +/- 16 mU/l following infusion of glucose plus gut extract (p less than 0.005). After injection of GIP antiserum in sufficient amounts to neutralize the GIP activity in the gut extract preparation, the additional insulin release due to the gut extract was reduced by only 30%. After complete removal of GIP from gut extracts by immuno-absorption, more than 50% of the incretin effect remained. These data suggest that the insulinotropic activity of rat gut extracts can only be partially related to GIP. The existence of additional insulinotropic gut factors which may also be released following oral glucose is postulated.