Cardiorespiratory and metabolic responses to steady-state dynamic leg exercise were studied in seven male subjects who performed positive and negative work on a modified Krogh cycle ergometer at loads of 0, 16, 33, 49, 98, and 147 W with a pedalling rate of 60 rpm. In positive work, O2 uptake increased with the ergometric load in a parabolic fashion. Net O2 uptake averaged averaged 220 ml-min-1 at 0 W (loadless pedalling), and was 75 ml-min-1 lower at the point of physiological minimum load which occurred in negative work at approximately 9 W. The O2 cost of loadless pedalling is for one-third attributed to the work of overcoming elastic and viscous resistance, the remaining part being due mainly to the work of antagonistic muscle contraction in the moving legs. Although at a given VO2, work rate was much higher in negative than in positive work, corresponding values for VE were similar, suggesting that the mechanical tension in working muscles is of little or no importance in the control of ventilation in steady-state exercise. Heart rate increased linearly with VO2 in both positive and negative work, with a steeper slope in negative work. Evidence is presented that none of the current definitions of muscular efficiency yields the true efficiency of muscular contraction in cycle ergometry, net efficiency calculation resulting in too low estimates, and work and delta efficiency calculations in overestimated values in the low-intensity work range, and in underestimated values in the high-intensity range.