The effect of dietary protein quality and quantity on fractional rates of protein synthesis (ks) and degradation (kd) in the skeletal muscle, liver, jejunum and skin of young growing chickens was studied. Chickens were either fasted overnight or were fed at frequent intervals, using continuous feeders, with equal amounts of a diet containing soya-bean meal as the sole protein source, unsupplemented, or supplemented with either lysine or methionine. Each of the three diets was provided at 2 or 0.9 x maintenance. On the higher intake, birds on the unsupplemented diet gained weight, lysine supplementation decreased and methionine supplementation increased body-weight gain (by -23% and +22% respectively). Birds fed at 0.9 x maintenance lost weight; supplementation with methionine or lysine did not influence this weight loss. None of the dietary regimens had significant effects on protein synthesis rates in any of the tissues, thus the mechanism whereby muscle mass increased in response to methionine supplementation appeared to be a decrease in the calculated rate of protein degradation. Similarly, on the 0.9 x maintenance diet the failure of the animals to grow appeared to be due to an increase in the rate of protein degradation rather than an effect on synthesis. Conversely, muscle ks was decreased in fasted chickens previously fed on the unsupplemented diet at 2 x maintenance, and in birds which had received the 0.9 x maintenance diet fasting resulted in a similar reduction in protein synthesis in muscle; ks in the liver and jejunum was also significantly decreased.(ABSTRACT TRUNCATED AT 250 WORDS)