In order to obtain the best method of weight reduction in sports with weight-classificationsystem, the present study was performed to detemine the changes in muscle proteinmetabolism during a 10-day weight reduction period in four college boxers.The main results obtained were as follows:1) Energy and protein intake decreased gradually during weight reduction period.2) Urinary excretion of 3-methylhistidine (3-Me), urinary nitrogen (Urinary-N) and ureanitrogen (Urea-N) in urine decreased significantly at the intermediate day of weightreduction period (IWR).3) At the end day of weight reduction period (EWR), Urinary-N and Urea-N increased, nitrogen balance tended to negative significantly, and body weight reduced remarkablyin comparison witht hose at IWR.However, the fluctuation of 3-Me value showed noclear tendency at EWR on each subjects.From these results, this study suggests that muscle protein degradation decreased at IWR incomparison with that at the before weight reduction period.The present author recommends for the intake amount of protein:1.5g/kg/day, and forcarbohydrate:5g/kg/day to prevent the muscle protein degradation when athletes want toreduce their body weight.