1. The incorporation of [(14)C]valine and [(3)H]threonine has been studied in mucosa of goldfish acclimatized to different temperatures. 2. [(14)C]Valine was incorporated more rapidly than [(3)H]threonine at acclimatization temperatures below 15 degrees; this distinction disappeared at higher temperatures. 3. The rate of amino acid incorporation rose as the environmental temperature was increased. 4. The number of valine residues in mucosal protein fell and the number of methionine residues rose as the acclimatization temperature was raised. 5. The concentration of free methionine was closely related to the amount in protein in goldfish mucosa; both increased at higher acclimatization temperatures. 6. An attempt is made to show that these changes are integrated with possible homoeostatic mechanisms in the goldfish intestine.
