Systemic sepsis is associated with reduced mesenteric blood flow and impairment of metabolic and barrier functions of the small intestine. A study was performed in the rat to investigate the effect of sepsis induced by caecal ligation and double puncture on intestinal absorption of leucine and proline in vivo. Absorption was studied 24 h after caecal ligation and puncture by measuring intestinal disappearance and circulatory appearance of intraluminal 3H-labelled amino acid over a 60-min study period. Peritonitis resulted in a significant increase relative to controls in the mean (s.e.m.) percentage of leucine (67.2(3.6) versus 18.0(3.5), P < 0.001) and proline (64.7(6.0) versus 15.1(3.7), P < 0.001) remaining within the small intestine. There were significant decreases in portal venous and femoral arterial concentrations of leucine and proline in animals with sepsis. Intestinal amino acid absorption is impaired in this model.