The uptake of amino acids by the non-gill epithelia of the mantle cavity of Mytilus edulis L. was studied and compared with uptake by the gills. Amino acid entry rates and the subsequent distribution of amino acids to the other tissues of the animals were studied using high-performance liquid chromatography and radiochemical techniques. Uptake via the non-gill epithelia lining the mantle cavity was separated from uptake via the gill by employing a preparation in which the gills were surgically removed. Amino acid uptake by such animals was compared with that of suitably sham-operated controls. In short-term experiments (up to 2h), transfer of substrate from the gills to other tissues of the animal is extremely limited. Amino acids taken up by the non-gill epithelia of the mantle cavity are rapidly transferred to deeper tissues. Roughly 25% of alpha-amino acids enter the animal via the non-gill epithelia. Estimates of total epithelial surface area for the gills and non-gill mantle epithelium are compared with entry rates of amino acid substrates via the two routes. The apparent densities of carriers for alanine and cycloleucine per unit area of surface are approximately equal for these two substrates. The density of taurine carriers per unit area of non-gill epithelium is apparently significantly higher than their density per unit area of gill epithelium. Finally, evidence is presented for differential sensitivity of taurine transporters in the non-gill epithelium to inhibition by alpha-amino acids.