An experiment was conducted to determine the effect of feeding ornithine in combination with alpha-aminoisobutyric acid (AIB), an inhibitor of arginase, on the regulation of polyamine synthesis in chicks. A total of 48 chicks with genetically elevated renal arginase activity was fed diets containing crystalline amino acids and 1% AIB with or without 2% ornithine. Feeding AIB reduced renal arginase activity, while renal and hepatic ornithine decarboxylase (ODC) activity increased. Feeding AIB plus ornithine caused no further reduction in renal arginase activity compared with that in chicks fed the AIB-supplemented diet. Renal and hepatic ODC activities, however, fell to below control levels. Renal, hepatic, and breast muscle ornithine concentrations increased substantially when ornithine was fed. AIB plus ornithine increased renal putrescine and spermidine concentrations. It was concluded that AIB could partially overcome the ornithine-induced inhibition of ODC activity. These findings support the hypothesis that dietary manipulation of precursor amino acids of polyamines in the presence of metabolites that induce ODC activity can influence tissue polyamine concentrations.