The effect of two dietary concentrations of proline (10.3 and 15.8 g/kg) on proline-metabolizing enzymes [pyrroline-5-carboxylate (P5C) reductase and proline oxidase], plasma and tissue free proline concentrations and growth were investigated in the 2- to 13-d-old pig. Diet had no effect on growth or enzyme activity. Diet had a significant (P less than 0.05) effect on the concentration of free proline in plasma, liver, intestine and muscle, but no effect in kidney. These data suggest that the magnitude and pattern of change of P5C reductase activity is not influenced by the concentration of proline in the diet. The lower plasma and tissue free proline concentrations in the piglets fed the basal diet compared with piglets fed the proline-supplemented diet and the lack of effect of diet on enzyme activity suggest there was inadequate proline in the basal diet, and those piglets were unable to increase proline synthesis to maintain normal proline concentrations.