The purpose of this study was to discuss the effect of swimming efficiency (e), buoyancy, and participation of aerobic and anaerobic metabolism on the relationship between swimming performance and energy metabolic capacity. The maximal oxygen debt (O_2 debt_max) and maximal oxygen uptake (Vo_2max) were measured after and during swimming in a swimming flume, as a parameter of anaerobic and aerobic capacity, respectively. Oxygen requirement (E) and swimming efficiency (e) were calculated from O_2 debt_max,Vo_2max, and swimming record. The subjects consisted of 44 freestyle swimmers with different sex, age and swimming record. The correlation between O_2 debt_max and 100m swimming speed was r=0.551 (p<0.01) for the group) of whole subjects, while the coefficients for the three groups classified in terms of e_1 (e in 100m swimming) were r=0.739 (p<0.01) for the low e_1 group (N=14, e_1=3.33), r=0.912 (p<0.01) for the intermediate e_1 group (N=17, 3.33