Experiments were conducted to determine the metabolic fate of threonine in chicks fed threonine-imbalanced diets. Threonine imbalance was produced by the addition of 3% serine to a threonine-limited diet, and prevented by the addition of 0.2% threonine to the diet. Serine decreased plasma and liver free threonine concentrations, and increased hepatic threonine dehydrogenase and threonine aldolase activities. All changes, including reduced food intake, appeared to occur within 1 day of feeding the imbalanced diet. Despite the decrease in free threonine concentrations and the increase in threonine aldolase and threonine dehydrogenase activities, net threonine catabolism was not markedly increased. This was evidenced by similar amounts of 14CO2 exhaled by chicks fed control and imbalanced diets containing L-[U-14C] threonine, and by similar growth of chicks that were forced-fed both diets to maintain equivalent food intake. It is possible that increases in threonine catabolism contribute to depressions of plasma and tissue threonine concentrations. However, the growth depression caused by serine-induced threonine imbalance is due to depressed food intake.