We investigated the neural regulation of the cardiac interval to an exercise demand signal and to a repeated exercise in 20 healthy human subjects. Electrocardiogram (ECG), muscle torque, and electromyogram (EMG) were simultaneously measured and their time relationships compared before and during the exercise. The R-R interval of ECG was directly increased by the exercise demand signal itself before the onset of EMG but not reflexly by muscle contraction. The cardiac interval decreased at the onset of exercise. Under the condition of repeated maximum eccentric training, the resting cardiac interval decreased prior to the exercise, whereas the brief increase in cardiac interval to the exercise demand signal remained unchanged. These results suggested that when autonomic nerve activity to the pacemaker is activated by the exercise demand signal, an initial effect of vagal nerve activity appears, and an effect of vagal nerve withdrawal and/or sympathetic nerve activity then appears. The responses of the heart and leg skeletal muscle at the onset of exercise are not synchronized, and the cardiac interval is controlled by vagal and sympathetic nerve activities to effect a transition to a high heart rate as quickly and smoothly as possible.