1. Changes in ventilation and cardiovascular variables which occur during exercise may be partly due to 'radiation' of activity in central neurones innervating exercising muscles to the respiratory and cardiovascular control areas. To test this hypothesis, we compared ventilatory and cardiovascular responses to two levels of steady-state exercise with each leg separately, in subjects with painless unilateral leg weakness. We assumed that exercise with a weak leg would require more central neural drive than the same level of exercise with the normal leg. 2. Ventilation during exercise with the weak leg was greater than with the normal leg (P less than 0.02). This was a result of greater tidal volume (Vt; P less than 0.005). There was a greater increase in heart rate (P less than 0.005), and systolic (P = 0.001) and diastolic (P less than 0.02) blood pressures during exercise with the weak leg compared to exercise with the normal leg. The increases in stroke volume and cardiac output during exercise were not different with the two legs. 3. These results support the hypothesis that ventilation, blood pressure and heart rate are influenced by the central neural drive to exercise.