Uptake of the neutral amino acid L-leucine into isolated rat intestinal brush border membrane (= BBM) vesicles and into a jejunal mucosa preparation as affected by the protein content of the diet was investigated. Adult rats fed either a high carbohydrate (HC) diet (11% protein) or a high protein (HP) diet (77% protein) for several weeks were used for the experiments. The time course of L-leucine uptake into BBM vesicles prepared from the small intestine of HC- or HP-rats was studied under conditions of an inwardly directed Na+-gradient and under Na+-equilibrium conditions. Furthermore, in one series of experiments the Na+-equilibrium was replaced by a K+-equilibrium. L-leucine uptake under Na+-gradient conditions displayed the overshoot phenomenon typically associated with Na+-gradient-dependent active transport processes in BBM vesicles and the overshoot in group HP exceeded that in group HC significantly. Under both Na+- and K+-equilibrium conditions L-leucine uptake into the BBM-vesicles also was faster in group HP. Finally L-leucine uptake into jejunal mucosa in group HP exceeded that in group HC, too. The results therefore indicate that Na+-dependent and Na+-independent transport of neutral amino acids across the intestinal brush border membrane adapts to the dietary protein level.