1. The absorption of a mixture of amino acids by the small intestine has been studied in vitro with mucosal slices from rat jejunum. The mixture contained eighteen amino acids and was used at a concentration of 9.5 mM in the incubation medium. The uptake of fifteen of the amino acids was followed in each sample with the aid of an amino acid analyser.2. The endogenous amino acid content of the slices decreased during preparation and further substantial losses occurred when the slices were preincubated for 30 sec before the addition of the amino acid mixture to the incubation medium.3. When the slices were incubated with the amino acid mixture for 4.5 min all of the amino acid studied were accumulated to give concentration ratios of approximately 2.0.4. The amino acids were taken up rapidly by the tissue so that on average 82% of the amino acids absorbed in 4.5 min had already accumulated after one min of incubation.5. The data for individual amino acids revealed no obvious competition between the various amino acids, and the extent to which the various amino acids were accumulated by the tissue was proportional to their concentrations in the mixture.6. When the Na(+) in the incubation medium was replaced by Li(+), the slices accumulated all of the amino acids studied to concentrations significantly higher than those in the incubation medium. However, the concentration ratios obtained with the Li(+) medium were all lower than those obtained with the Na(+) medium and averaged slightly more than 1.5.7. When the Na(+) in the incubation medium was replaced by K(+) there was little amino acid accumulation, although concentration ratios significantly greater than one were achieved for aspartic acid, glycine, histidine and methionine.