The purpose of this study was to compare the oxygen cost of running as it relates to speed of running among the following four groups: trained male distance runners, trained female distance runners, untrained but active men and women. Each subject was given a series of treadmill tests during which Vo2was measured at submaximal work loads. The linear regression equation was utilized to compute the relationship between Vo2and running speed for each group. The results indicated that the rate of increase in Vo2for a given increase in running speed could be represented as a straight line and was the same for all groups (P > .05). The trained male runners had a significantly lower Vo2(P < .05) than those of the other three groups at any measured speed. The trained females and untrained males had significantly lower Vo2s than the untrained females (P < .05) at any of the given range of speeds. No significant differences were observed between the untrained men and trained women (P > .05). It was concluded that-there were differences in the oxygen cost of running not only between the trained and untrained groups but also between males and females.