An examination was made of the comparability of one- and two-leg exercise performed pedaling a stationary bicycle ergometer. The pattern of force exerted on both cranks was examined by means of a specially adapted ergometer which is described. The mean of the peak force in each cycle (MPF) was linearly related to work load (W) in both forms of exercise, and if account was taken of the doubled work output in two-leg cycling there was no significant difference between the MPF/W relationships; these are given by the equations one-leg: MPF (kg) = 11.23 + 0.065 (W in kpm/min) two-leg MPF (kg) = 10.76 + 0.032 (W in kpm/min). Calculation from the force records of the work performed on the cranks (WCR net) showed good agreement (r = 0.98, P less than 0.001) with the work load set on the ergometer. Analysis of the proportion of work done in leg extension and flexion phases of cycling revealed no differences between one- and two-leg exercise or between the right and left legs. The majority (approximately 80%) of Wer net being performed in leg extension is described by: Wer net (extension)) = 10.6 +/- 0.8 (W cr net total). In one-leg exercise (W greater than 900 kpm/min) the variation in rotation speed during a normal cycle ranged from +20 to --30% of the mean speed compared with +/- 10% in two-leg exercise.