1. Studies have been made of jejunal absorption rates in vivo in the rat of the stereoisomers of alanylphenylalanine, leucyl-leucine and glycyltryptophan. Absorption rates of l-alanyl-l-phenylalanine were about 200 times those of d-alanyl-d-phenylalanine, l-leucyl-l-leucine about 24 times those of the dd-isomer, and glycyl-l-tryptophan 5 times those of glycyl-d-tryptophan. The mixed ld- and ld-isomers were absorbed at intermediate rates.
2. Absorption rates were positively correlated with the rate of hydrolysis of each dipeptide by homogenates of rat intestinal mucosa. The transport rate and rate of hydrolysis of glycyl-d-tryptophan, d-alanyl-d-phenylalanine and d-leucyl-d-leucine were significantly greater in the ileum than in the jejunum.
3. When given by stomach tube the most slowly absorbed dipeptides, d-alanyl-d- phenylalanine, d-leucyl-d-leucine and glycyl-d-tryptophan were the only ones to be excreted in significant amounts in the urine, showing that they were absorbed as the entire molecule and were resistant to hydrolysis by tissue peptidases.
4. There was a close inverse correlation between the rates of transport by rat jejunum of glycine, d-alanine, d-leucine, d-phenylalanine, d-tryptophan, d-alanyl-d- phenylalanine, d-leucyl-d-leucine and glycyl-d-tryptophan and the molecular weight of each compound, suggesting that diffusion mechanisms play an appreciable part in jejunal absorption of these compounds. No such correlation was found in the case of the ileum.