Six, highly trained male college taekwondo athletes were studied after the nine days of fast weight reduction program using diet control, exercise, and perspiration in an attempt to determine the effects of weight reduction on cardio-respiratory function and aerobic capacity. The LBM reduction percentage was averaged to 39% of the total weight reduction. The precent fat decreased 30%, and the body fat also decreased 57% in the suprailiac part. The grip strength was unaffected, although back strength was increased 21% after the weight reduction. Trunk flexion decreased 5%, whereas trunk extension was barely increased. The heart rate was decreased 10 beats/min during the bicycle ergometer submaximal exercise, and VO₂ max, VE and HLa accumulation were significantly decreased during the maximal exercise. The blood glucose values were increased during the maximal exercise, and total protein values were also increased 21% and 13% during the rest period and maximal exercise respectively. MCH and MCHC increased from 4% to 11%. In conclusion 9 percent of fast weight: reduction was belived to decrease the cardiorespiratory function and aerobic capacity due to the reduction of Hb and blood corpuscules.