1. The serosal transfer of the following eight amino acids: threonine, alanine, serine, histidine, valine, methionine, phenylalanine and leucine, was measured using everted sacs of anterior intestine taken from goldfish acclimatized to 8 degrees C and incubated at 25 degrees C.2. All eight amino acids were actively transported and the serosal transfer correlated with the steady potential (P < 0.001) and with the amino acid-evoked potential (P < 0.05) measured on the same preparations.3. The goldfish rectum actively transported alanine and the steady potential was raised when alanine bathed the mucosa of the everted preparation.4. L-aspartic acid was partly transaminated to alanine by the goldfish anterior intestine; the rectum transaminated alanine to an unidentified amino acid which might have been serine, asparagine or glutamine or some mixture of these three.5. It is suggested that L-amino acids increase the ease by which sodium enters the mucosal cell but that it is the rate at which this sodium is transported across the basal membrane which determines the net serosal transfer of amino acids.