Fractional rates (%/day) of muscle protein synthesis and degradation of the genotypes Dw/Dw and dw/dw of male White Plymouth Rock chickens were determined by measuring the output of N tau-methylhistidine (N tau-MH) in the excreta at 2, 4, and 8 weeks of age. The fractional growth rate of dw/dw was significantly lower (P less than 0.05) than that of Dw/Dw at 2 weeks of age but not at 4 and 8 weeks of age. No significant differences in the degradation rate (Kd; %/day) were found at any age. A significant difference (P less than 0.05) between genotypes in the rate of synthesis (Ks; %/day) was found at 2 weeks of age (Dw/Dw = 11.8, dw/dw = 9.9) but not at 4 and 8 weeks of age. These results suggest that the dw gene has a depressing effect on the synthesis rate of muscle protein, and the difference between genotypes in the growth rate at the early stage is a reflection of this effect.