Whole-body protein turnover rates were measured in chicks fed control, histidine-free (-His), and methionine plus cystine-free (-Met, Cys) diets. After chicks were fed the experimental diets ad libitum for 7 days, they were injected with a massive dose of L-[4-3H]-phenylalanine (40 microCi/100 g body weight) and rate of protein synthesis was estimated from the incorporation of phenylalanine into protein. The rate of protein degradation was estimated as the difference between the synthesis and growth rates of protein. In chicks fed the amino acid-devoid diets, the fractional synthesis rates of protein (FSR, percent per day) were significantly lower than those in control chicks, whereas fractional degradation rates of protein (percent per day) were constant for all dietary treatments. The FSR in -Met, Cys chicks was lower than that in -His chicks and the lower FSR in -Met, Cys chicks seemed to result in more body protein loss. The RNA/protein ratio of chicks was almost the same in all treatments, but the protein synthesized per unit RNA varied depending on the dietary treatments. The lower amount of protein synthesized per unit RNA in -Met, Cys chicks was postulated to result from inhibition of protein synthesis by a shortage of available methionine associated with a higher degradation rate of methionine itself. These results indicated that different growth responses between -His and -Met, Cys chicks were primarily caused by the difference in the rates of protein synthesis.