A study was conducted to investigate the influence of maximal running speed, aerobic and anaerobic components and muscular endurance of the lower limbs on the decrease in running speed during 400 m running. Fifteen track and field athletes (400 m sprinters, decathletes and middle-distance runners) participated. The subjects were used to obtain data on changes in running speed during 80 m and 400 m running, maximal O_2 intake, O_2 debt and isokinetic muscular endurance. The time taken to cover 400 m was negatively correlated with maximum O_2 (r=-0.558; p<0.05) and muscular endurance of hip flexion (r=-0.521; p<0.05). Running speed over a distance of 400 m began to decrease after 80 m, and kept decreasing until the finish. The change in running speed after 80 m was expressed as a linear regression equation, and the regression gradient was interpreted to be an index of the decrease in running speed. There were significant correlations between the muscular endurance of hip flexion and extension respectively (r=-0.666; p<0.01, r=-0.517, p<0.05). These results suggest that the muscular endurance of hip flexion affects the decrease in running speed during 400 m running, and also performance.