1 A plethysmographic preparation of cat jejunum was used to measure changes in tissue volume and capillary filtration coefficient (CFC), simultaneously with measurements of arterial and venous pressures, and total blood flow. 2 Secretin was infused and injected intravenously and also infused intra-arterially in relatively small doses. Probable resulting blood concentrations were compared with those determined under physiological conditions in other investigations. 3 By intravenous or intra-arterial infusion, secretin caused increases in CFC, indicating an increased functional exchange vessel area, and increases in jejunal volume, indicating increased vascular capacitance. The jejunal blood flow increased whilst the blood pressure remained essentially unchanged. 4 By intravenous injection, secretin caused rises in jejunal volume and reductions in calculated jejunal vascular resistance over the same dose range. Effects were statistically significant at 500 mu/kg and higher doses caused reductions in systemic arterial pressure. 5 The calculated peak blood concentrations of secretin resulting from the lower doses used in this investigation were of the same order of magnitude as those determined under physiological conditions in man. 6 It is possible that at physiological concentrations secretin causes an increased functional exchange vessel area in the small intestine, and may also increase the total blood flow through this tissue.