Turtles (Pseudemys scripta elegans) and alligators (Alligator mississipiensis) were fed fish, casein, or free amino acid mixtures, or were injected with free amino acid mixtures, quantities in each case being equivalent to about 42 mmoles of amino acids/kg body weight. The experiments were designed to determine whether protein synthesis normally precedes catabolism of the amino acids absorbed from digesting food. Both animals disposed of free or food-derived amino acids more rapidly than could be accounted for by catabolism alone, but the transient increases in turtle plasma concentrations consisted mostly of essential amino acids, whereas the alligator plasma showed little increase in essential amino acids and considerable rises in four nonessential amino acids, glutamine, glutamic acid, glycine and alanine. Despite some species variation, these two species with low metabolic rates apparently utilized protein synthesis to dispose of amino acid mixtures prior to the necessarily slow process of catabolism for energy.