OBJECTIVE The aim of this study was to investigate the influence of stimulus intensity on the waveform of sympathetic skin response (SSR). The origin of the SSR waveform was discussed. METHODS A total of 12 SSRs from palm skin were analyzed in 40 normal subjects. SSR was evoked by magnetic stimulation to the neck at 3 different intensities (10, 35, and 70% of the maximum output). The 3 stimuli were repeatedly applied in 4 separate sets. Waveforms were classified as either the P type, in which the positive component was larger than the negative one, or the N type, in which the negative component was larger than the positive one. Amplitude values of peak-to-peak, the first negative (N1), and subsequent positive (P1) components were compared among the 12 responses. RESULTS When the stimulation was increased, the SSR size became larger. The P type SSR was most frequently found in the maximum stimulation. Strong stimulation generally produced a large P1. Only the N type SSR exhibited a large N1 response to the maximum stimulation. There was a significant, negative correlation between N1 peak duration (difference between the SSR onset and N1 peak latencies) and P1 amplitude. CONCLUSIONS These results suggested that strong responses probably produced a P type SSR with a large P1 component. In this condition, a rapid change of potential from negative to positive prevented any correlation between N1 amplitude and the magnitude of the response. In the N type SSR, however, a large N1 was associated with a strong response. These findings can be explained by a newly presented theory on the neurophysiological origin of the negative and positive components, based on a model of equivalent current dipole dependent on the Na+ concentration gradient. CONCLUSIONS The present study provided some suggestions on the neurophysiological mechanism of SSR waveform.