In six young men, heart rate and arterial mean blood pressure responses to the onset of light dynamic exercise 99 W (range 59-138) on a stationary bicycle were followed during partial neuromuscular blockade with tubocurarine. Tubocurarine was used in order to accentuate the central nervous (central command) influence on the cardiovascular variables and reduced hand-grip strength to 42% (36-47) of control. At the onset of exercise heart rate increased immediately and similarly with and without neuromuscular blockade. Mean arterial blood pressure remained constant during the first 6 s of control exercise and then increased. With tubocurarine a decrease of 9 mmHg (3-12) was seen during the first 6 s (P less than 0.01) before blood pressure increased. The similar heart rate responses seen with and without neuromuscular blockade indicate that central command has little influence on this variable at the onset of dynamic exercise. The constant blood pressure at the onset of control exercise suggests that the immediate changes in cardiac output and peripheral vascular resistance, respectively, are accurately matched. The decrease in blood pressure at the onset of exercise with tubocurarine suggests that central command stimulates vasodilatating nerves to arterioles in the working muscles.