The aerobic energy cost and certain circulatory and ventilatory responses were studied in 2 human subjects performing work on a bicycle ergometer and employing either concentric (Wcon) or eccentric (Wccc) muscle contractions. Work was performed at intensities ranging from 15–130 Watt and at various speeds (range 20–100 rpm). Oxygen consumption depended upon both work intensity and speed and demonstrated large differences between Wconand Wccc. Heart rate and ventilalation corresponded closely to oxygen consumption but showed definite tendencies to be elevated in Wccc. Calculation of work efficiency as oxygen consumption in excess of values during free-wheeling activity evidenced higher efficiency with increasing pedaling frequency.