The interrelationships between dietary pyridoxine and free plasma amino acids in young chicks were studied with two different diets. Chicks fed isolated soybean protein supplemented with less than 1.0 mg pyridoxine-HCl/kg diet had reduced body weight gain and feed consumption and a higher mortality rate (19%) than those fed 1 or 5 mg pyridoxine-HCl/kg. A casein diet required greater supplementation (2.0 mg pyridoxine/kg) to prevent reduced body weight gain and depressed feed intake. The concentrations of free plasma taurine, threonine, and serine were significantly lower in chicks fed the isolated soybean protein diet supplemented with less than 1.0 mg pyridoxine-HCl/kg, while those of isoleucine, valine, glycine, and ornithine were significantly higher than in chicks fed greater levels of pyridoxine. Supplementation of casein diets with less than 2.0 mg resulted in a significant accumulation of threonine, glycine, cystathionine, and ornithine. Valine was significant higher in chicks fed casein supplemented with only .6 mg pyridoxine-HCl/kg. Feeding isolated soybean protein or casein diets without any pyridoxine supplementation resulted in the dealth of 95% and 100% of the chicks, respectively. Pair feeding a group fed the isolated soybean protein diet supplemented with 1.0 mg to a group receiving .5 mg pyridoxine-HCl/kg resulted in a similar body weight gain and concentration of amino acids in the plasma.