Experiments were conducted to estimate the threonine requirement of pigs weighing 5 to 15 kg and to determine the effects of dietary excesses of methionine, threonine, and lysine in weanling pigs and chicks;Feeding .70% dietary threonine maximized gain/feed (P .10) pig performance or plasma urea N. In another experiment, a 15% crude protein diet, was supplemented with .40% methionine and/or .15% threonine. Amino acid additions did not affect performance of pigs, but pigs fed supplement threonine had the lowest plasma urea N concentrations;Performance and plasma urea N of weanling pigs fed a practical diet were not affected by either lysine and/or threonine supplementation. Dietary lysine supplementation increased plasma lysine (P < .01) and decreased plasma threonine levels (P < .01), but this latter effect was greater in pigs fed the diet supplemented with threonine than in those fed the unsupplemented diet (interaction, P < .05). In an experiment with chicks, there was an interaction of dietary lysine and threonine on growth (P < .01) and on efficiency of feed utilization (P < .05). Increasing dietary lysine depressed growth and gain/feed of chicks fed the lowest dietary threonine level but improved growth and efficiency of those fed the two higher dietary threonine levels. Increasing dietary lysine concentrations increased plasma lysine levels (P < .01) and decreased plasma threonine (P < .01). Plasma threonine levels were increased by dietary threonine supplementation (P < .01);The results of these experiments suggest that the threonine requirement of the weanling pig is about .70% of a diet containing 1.15% lysine and that total sulfur amino acid concentrations as high as .97% do not depress performance of young pigs. High plasma lysine concentrations seem not to be involved in mediating feed intake depression in pigs fed an excess dietary lysine concentration (1.4%). Threonine imbalance can be induced in chicks fed diets adequate in protein and marginally deficient in threonine by increasing the dietary lysine concentration, and this imbalance can be alleviated by further threonine supplementation.