Tests of performance on a bicycle ergometer have routinely been standardized with pedaling frequencies of up to 60 rpm. Only rarely have higher speeds been used. It may be hypothesized that a higher [(V)\dot]O₂\dot VO_2 , more closely approximating the maximum attainable by treadmill running, may be achieved in bicycle ergometry if higher pedal frequencies are used so that premature fatigue of the leg musculature does not obscure the comparison. Even in treadmill running it remains equivocal which combination of speed and grade of running will produce a maximum [(V)\dot]O₂\dot VO_2 . Five male subjects performed maximally at pedal frequencies of 60, 80, 100 and 120 rpm and running speeds of 6.0, 6.5, 7.0 and 7.5 mph, on a bicycle and treadmill respectively. Power output on the bicycle and increasing grade sequence on the treadmill were maintained constant for each speed investigated. The highest [(V)\dot]O₂\dot VO_2 attained in each test was measured and compared both within and between the modes of testing. It was confirmed that peak [(V)\dot]O₂\dot VO_2 during bicycle ergometry is significantly less than [(V)\dot]O₂\dot VO_2 max attained in treadmill running. Pedal frequencies of 80 and 100 rpm produced optimal [(V)\dot]O₂\dot VO_2 's in ergometry. No differences were noted between [(V)\dot]O₂\dot VO_2 's determined at any speed of treadmill running. Subjects preferred 60 or 80 rpm in ergometry and 6.0 or 7.0 mph in treadmill running; 120 rpm and 7.5 mph in bicycling and treadmill running, respectively, were generally disliked.