In this study we evaluated the effect of two different doses of secretin on portal haemodynamics (by pulsed Doppler associated with real time ultrasound) in 24 healthy humans. In 12 subjects (group A) we administered an intravenous dose of 75 clinical units of secretin and in the remaining 12 (group B) a dose of 20 CU. In all subjects the following parameters were studied before, during, and for 10 minutes after secretin administration: (a) calibre of the portal vein, (b) mean velocity of portal venous flow, and (c) volume of portal venous flow. In three subjects in each group we also evaluated the changes in flow in the superior mesenteric artery. Secretin injection induced a slight increase in both groups in comparison to basal values of portal vein calibre (mean of maximal per cent increase +25% in group A, not significant, and +16.7% in group B, not significant) and a noticeable increase of mean velocity (mean of maximal per cent increases +61.4% in group A, p less than 0.005, and +65.4% in group B, p less than 0.01) and flow volume (mean of maximal per cent increase +127% group A, p less than 0.005, and +114% group B, p less than 0.005). The magnitude of the haemodynamic changes did not differ significantly between the two groups. Doppler investigation of the superior mesenteric artery showed a marked increase of flow velocity (mean of maximal per cent increase +218% in group A and +246% in group B) and flow volume (mean of maximal per cent increase +276% in group A and +311% in group B). These data suggest that secretin has an appreciable vasoactive effect and induces a significant increase in portal venous flow even at doses much lower than those necessary for a maximal stimulation of exocrine pancreatic secretion.