We investigated the effects of medium (MCT) and long (LCT) chain triglyceride test meals on gall bladder contraction (using ultrasonography) and small intestinal bile acid concentrations and ileal flow rates (using intubation techniques) in normal individuals. Comparing the effects of ingesting medium chain triglyceride and long chain triglyceride meals, the gall bladder volume was reduced by 23.4 +/- 33.6% and 59.0 +/- 17.1% respectively (mean +/- SD, p less than 0.01, n = 13), the jejunal bile acid concentrations increased significantly only after the ingestion of long chain triglyceride (p less than 0.05, n = 5), while the volume of the postprandial jejunal contents was not significantly different (540 +/- 150, 522 +/- 169 ml, medium chain triglyceride vs long chain triglyceride, p = 0.2, n = 5). The postprandial colonic inflow was 48.5 +/- 12.5 and 123.4 +/- 35.2 ml/h (medium chain triglyceride vs long chain triglyceride, p less than 0.01, n = 5). Thus medium chain triglyceride, compared with long chain triglyceride, produces a smaller input of bile acids into the small intestine and a smaller volume of fluid delivered to the colon. These observations may be relevant to the beneficial effects of medium chain triglyceride substitution for long chain triglyceride in the treatment of diarrhoea in patients with small intestinal disease.