The validation of the urinary excretion of N tau-methylhistidine (N tau-MH) by quail as an index of the muscle protein turnover rate was tested using the criterion of the rate of recovery of radioactivity in urine following an intraperitoneal dose of L-[3-14C] methylhistidine. A genetic study on muscle protein turnover in quail was conducted using three genetically diverse lines (LL, large body size; SS, small body size; RR, random-bred control line) selected for body size. When L-[3-14C] methylhistidine was administered to 20-week-old male and female coturnix quail by direct intraperitoneal injection, approximately 90% of the L-[3-14C] methylhistidine was recovered by 96 hr postinjection. Recoveries were low in the egg and muscle. These results show that N tau-MH released from myofibrillar protein is not reutilized and the excretion of N tau-MH is a satisfactory index of muscle protein breakdown. In all lines, the amount of urinary N tau-MH excretion and fractional synthesis (Ks) and degradation (Kd) rates at the high growing period were higher than those at the low growing period. The Ks and Kd are significantly different among selected lines at both 3 and 6 weeks of age. At 3 weeks of age, the fractional rate of synthesis of the LL line (13.2%/day) was higher than that of the RR line (11.5%/day), whereas the SS (8.1%/day) was lower than that of the RR line (11.5%/day). The fractional rates of degradation of both the LL line (4.1%/day) and the SS line (5.6%/day) were lower than that of the RR line (7.0%/day) at 3 weeks of age. From these results, it was recognized that selection for body size gave rise to the changes in the muscle protein turnover rate.