This experiment was designed to quantify changes in utilization of Gln and Leu by the gut wall as a result of changes in the starch supply to the duodenum. Four dairy cows fitted with cannulas in the rumen and the distal duodenum were adapted for 3 wk to starch infusion, either into the rumen (600 g/d of flaked maize) or into the duodenum (300 g/d of flaked maize plus 300 g/d of maize meal), in a 2 x 2 crossover design. Absorption and elimination kinetics and the relative bioavailability of Gln and Leu were measured during wk 4 and 5. After infusion of 50 g of Gln or 10 g of Leu into the duodenum or jugular vein, blood samples were taken from the jugular vein at 0.5-h intervals up to 4 h after infusion. Concentrations of Gln and Leu in plasma fitted best to an open, one-compartment model (duodenal infusion) or to an open, two-compartment model (i.v. infusion). Both amino acids were rapidly absorbed; half-life times were less than 20 min. The amount of Gln trapped in the splanchnic bed was higher than the amount of Leu trapped in the splanchnic bed. Site of starch infusion did not affect the relative bioavailability of amino acids.