Nitrogen flux across the splanchnic bed is altered following operation, injury, and sepsis, but the individual contributions of gut and liver and their interrelationships remain undefined. Since more than 60% of whole blood amino acid nitrogen is transported as glutamine and alanine, we determined the flux of these amino acids across the gastrointestinal tract and liver in splenectomized, awake dogs during a control period and a 2 and 4 days following a standard laparotomy. Blood flow was measured in all studies and substrate flux calculated from flow and arteriovenous and portovenous concentration differences. Portal blood flow decreased by 25% following operation from a control value of 26 +/- 2 ml/kg body weight . min to 19 +/- 2 (P less than 0.05). Total hepatic blood flow did not change significantly after operation, but the individual contributions of the hepatic artery and portal vein were altered; hepatic artery flow increased from a control value of 10 +/- 1 ml/kg . min to 23 +/- 3 (P less than 0.001). Glutamine uptake by teh gastrointestinal tract nearly doubled from a control value of 0.75 +/- 0.16 microM/kg . min to 1.31 +/- 0.13 (P less than 0.05) on postoperative day 2. This increase in flux occurred despite a diminished arterial concentration and a reduced portal blood flow, indicating that extraction of glutamine by the gastrointestinal tract was not primarily dependent on increased arterial concentration. Alanine, on the other hand, was released by the gut at a rate of 1.97 +/- 0.37 microM/kg . min in controls and decreased to 0.81 +/- 0.13 microM/kg . min (P less than 0.05) in dogs that had operation. Glutamine was released by the liver in control dogs at a rate of 1.59 +/- 0.59 microM/kg . min but switched to an organ of slight glutamine uptake (0.31 +/- 0.31, P less than 0.01) on postoperative day 2. Alanine uptake by the liver doubled from 2.94 +/- 0.29 to 5.46 +/- 0.63 microM/kg . min (P less than 0.05) following surgical stress. The gastrointestinal tract plays an active metabolic role in the processing of amino acids following operation and may be a key regulatory of interorgan substrate flux following injury and infection.