Experiments were conducted on chicks and rats to determine whether hepatic threonine dehydrogenase activity is modified by the consumption of a threonine-imbalanced diet and to determine the tissue distribution of this enzyme. Threonine imbalances were created by supplementing basal diets with branched-chain amino acids (6 g/100 g diet for chicks) or a mixture of indispensable amino acid (5.6 g/100 g diet for chicks and 5.4 g/100 g diet for rats). Chicks fed threonine-imbalanced diets consistently had twice the hepatic threonine dehydrogenase activity of those fed the basal diet when measured in one experiment at 24 h and in two experiments at 216 h (P < 0.05). Rats received the experimental diets for 12 or 24 h in one experiment and for 12, 24, 72 or 168 h in another experiment. In the first experiment, rats fed the threonine-imbalanced diet had significantly greater hepatic threonine dehydrogenase activity (P < 0.05) at 12 h but not at 24 h. In the other rat experiment, threonine dehydrogenase activity in the rats fed the threonine-imbalanced diet was significantly greater than in controls at 72 h, but tended to be lower at 168 h, which coincided with the adaptation of the rats to the imbalanced diet. Threonine dehydrogenase activity was widespread in tissues of both species. The results indicate that alterations in hepatic threonine dehydrogenase activity occur in chicks and rats subjected to threonine imbalance.