Our purpose was to study the effect of arm, leg, and combined arm-leg ergometry on the oxygen uptake (Vo2), cardiac output (Q), ventilation, and anaerobic threshold (AT) of three healthy men. At submaximum work intensities, Vo2 was not significantly different in the three tasks, but differences were observed for heart rate, ventilation, and Q. The AT was reached at progressively higher work rates in arm, leg and combined arm-leg ergometry, respectively. The Vo2 max in arm ergometry averaged 68 percent of the Vo2max in leg ergometry and 60 percent of Vo2 max in combined arm-leg ergometry. Two subjects with Vo2max's less than 45 ml/kg-min had a mean Vo2max in combined arm-leg ergometry 19 per cent higher than in leg ergometry. A third subject, with a Vo2max greater than 50 ml/kg-min, showed no change. Differences in Vo2max were primarily due to the differences in Q. Skeletal muscle blood flow appears to be a critical factor in the limitation of Vo2max in arm or leg ergometry.