Changes in plasma levels of amino acids derived from fed protein were determined by feeding crocodilians (Caiman crocodilus crocodilus and Alligator mississipiensis) 7.5 g protein/kg body weight and by monitoring the plasma free amino acids for several days. Zein and several other vegetable proteins produced no rise in plasma amino acids and were excreted intact in the feces. Casein and fish muscle were rapidly digested but produced little rise in plasma amino acids, and the increases showed no relationship to the composition of the protein fed. Gelatin feeding led to large increases in plasma amino acids that persisted for more than a week, and the resulting pattern was nearly identical to the composition of gelatin with the exception of aspartic and glutamic acids, and several animals died. Equivalent quantities of fish muscle protein were assimilated without difficulty by the crocodilians. Endogenous protein secreted into the gut apparently contributed little to the amino acid mixture absorbed.