Experiments were conducted with crossbred chicks to examine the growth response to a vitamin when its precursor amino acid was in either deficient or adequate supply in the diet and vice versa. The tryptophan-niacin and methionine-choline interrelationships were thus examined. No growth responses to dietary niacin or choline were observed during severe tryptophan or methionine deficiency, respectively. However, when adequate levels of the precursor amino acids were present, marked gain and gain/feed responses to these vitamins occurred. In contrast, gain and feed efficiency responses to tryptophan or methionine occurred regardless of the chick's niacin or choline status, respectively.