Several experiments were conducted to identify the limiting amino acids in a 22% corn-soybean meal diet for poults from one to three weeks of age. Additional studies were designed to investigate changes in muscle composition and protein turnover in response to essential (EAA) and nonessential amino acid (NEAA) supplementation of the low protein diet. Developmental changes in muscle metabolism as well as differences between muscles were also examined. Protein synthesis was determined using a ¹⁴C-tyrosine emulsion technique. Protein degradation was calculated as the difference between synthesis and deposition rates. A 30% protein diet served as a control in all experiments. Methionine was confirmed as the first limiting amino acid. Individual deletion of EAA from a mixture added to the 22% protein diet indicated deficiencies of lysine, threonine and valine. Although lysine was more deficient than valine, the valine deficit was exacerbated by high dietary leucine and isoleucine levels. A dietary level of 1.25% valine was inadequate to support optimum growth, inferring a higher requirement than current recommendations. The addition of methionine, lysine and threonine and valine to the 22% protein diet supported maximum growth but feed efficiency remained depressed. Improvements in performance associated with NEAA supplementation indicated a crude protein deficiency in the 22% protein diet. The weights of the pectoralis and gastrocnemius muscles varied directly with body weight. Increases in pectoralis weight were primarily the result of expansions in DNA-unit size. Changes in DNA-unit size and number of the gastrocnemius could not be demonstrated despite consistent alterations in muscle mass. A supplement of methionine and lysine significantly decreased pectoral synthesis rate although protein synthesis tended to decline with any combination of added amino acids. The fractional synthesis rate in the gastrocnemius was relatively stable regardless of diet although a decrease was noted with methionine supplementation. Therefore, alterations in the fractional rate of protein deposition in both muscles were primarily attributable to fluctuations in degradation. Protein deposition was markedly reduced with methionine and lysine supplementation. However, the fractional and absolute rates of protein deposition were maximized by the combined addition of methionine, lysine, threonine and valine, concurring with body weight gain results. Thus, while equal rates of deposition were obtained with the 30% protein diet and the EAA-supplemented 22% protein diet, the latter represents a considerable reduction in energy expenditure for protein turnover. / Ph. D.