The time courses of the heart rate (HR) changes were studied in six healthy male subjects who performed step changes from rest to light dynamic leg exercise (50 W) in the sitting position during a) control, b) parasympathetic blockade (atropine, 2-2.5 mg iv), c) beta-adrenergic blockade (propranolol, 10 mg iv), and d) during combined blockade with both drugs. During the control and beta-blockade experiments all subjects showed an immediate, rapid increase in HR, reaching a peak value after about 10 s, whereafter an equally rapid transient drop by 10-20 beats/min took place reaching the lowest values about 17 s after the onset of work. HR then again increased to reach a steady-state level within 60-90 s. In the atropine experiments this response pattern was not evident at all, whereas in the experiments with combined blockade it could be distinguished to a small, but significant degree. It is concluded that the observed fluctuations of HR at the onset of light dynamic exercise can be explained by a rapid vagal withdrawal, followed by a transient increase in vagal tone.