We studied the sudomotor potential in the palm of the hand, evoked by magnetic stimulation of the neck, in 15 normal subjects and five patients with dysautonomia. The potentials consisted of two positive peaks (S1, S2), with mean (+/- SD) onset latencies of 408.5 +/- 31.8 and 619 +/- 61.4 msec. The sudomotor potential was clearly obtained only from the palm and was influenced by stimulus intensity, coil position, current direction in the coil, atropine or pilocarpine iontophoresis, and motor effort. With this method, we estimated the postganglionic sympathetic conduction time and the central reflex time, the mean values (+/- SD) of which were 408.5 +/- 31.8 and 323.3 +/- 45.2 msec. In three patients with severe dry skin, the sudomotor potential and the sympathetic skin response were not obtained. A patient with OPCA had a prolonged central reflex time, and a patient with alcoholic polyneuropathy showed a delayed sudomotor potential. Sudomotor potential is a noninvasive and useful method for evaluating sympathetic sudomotor function in patients with central and peripheral nervous system disorders.