The transport of labelled (hot) and non-labelled (cold) serotonin (5-HT) into the mesenteric venous circulation was studied after instillation of test solutions into an isolated jejunal loop of anaesthetized rats. After instillation of [3H]H2O and [14C]5-HT there was an almost parallel appearance of the isotopes in mesenteric venous blood. After instillation of 5-HT a marked early increase of the total amounts of cold 5-HT was observed in mesenteric veins compared with animals instilled with saline only. In a third type of experiment the label was detected in mesenteric venous whole blood after instillation of [3H]5-HT into the gut lumen. After hydrolysis of blood cells and protein precipitation the samples were fractionated and determined for 5-HT and metabolites. Only 5-HT was detected in these fractions. The label was present within 5-HT peaks in three out of eight animals. The experiments indicate rapid transport of 5-HT (or metabolites) across the rat jejunal mucosa. These substances may be bound to a binding protein in platelets since the isotope was detected in whole blood but more seldom in supernatants after hydrolysis and precipitation.