The purpose of this study was to compare the physiological responses of highly trained middle (MD) and long distance (LD) runners during treadmill running. The oxygen uptake (VO2) of 74 elite runners (42 MD and 32 LD) was measured during treadmill running at several speeds (201, 241, 282, and 322 m/min at 0% grade) and at maximal effort. The mean VO2max (ml/kg X min) of the LD runners (76.9) was significantly higher (p less than 0.01) than the value for the MD group (68.9). At each running speed, the relative oxygen costs (%VO2) was lower (p less than 0.01) for the LD group averaging 8% less across the four running speeds. The slopes of the relationship between submaximal VO2 (ml/kg X min) and running speed of 0.183 and 0.216 for the MD and LD groups, respectively, were not significantly different. The relationship between running performance, maximal treadmill running time (TRT), and VO2 was studied for each group. VO2max was more highly correlated with running performance in the MD group (r = 0.70) than in the LD group (r = 0.32) although the standard errors of estimate were similar for both groups. Results of this investigation demonstrate that there are differences in the metabolic characteristics of the MD and LD runners as classified in this study.