Abstract Experiments were conducted to determine the effect of dietary proline on the activity of enzymes of proline metabolism. The omission of proline from the diet for young chicks was without effect on kidney arginase, proline oxidase, Δ1-pyrroline-5-carboxylic acid dehydrogenase; and kidney and liver ornithine-δ-transaminase and Δ1-pyrroline-5-carboxylic acid reductase activities. The lack of dietary proline, however, consistently increased the activity of liver Δ1-pyrroline-5-carboxylic acid dehydrogenase. It is proposed that this latter activity as assayed in vitro may represent in part the activity of an enzyme involved in the synthesis of proline from glutamic acid.