Skin sympathetic nerve activity (SSNA) from the tibial nerve and event-related brain potentials (ERPs) were recorded simultaneously during auditory oddball count paradigms in ten healthy subjects to elucidate the relationships between the autonomic nervous system and the cognitive process. After target tones, SSNA bursts were observed more frequently than after non-target tones during oddball paradigms. When subjects ignored the series of tones, there was no significant difference between the incidence of SSNA bursts after rare tones and frequent tones. The amplitudes of SSNA bursts elicited after the target tones were higher than those elicited after the non-target tones. The P300 latencies of the target trials with SSNA bursts were significantly shorter than those of the target trials without SSNA bursts. The averaged ERP wave forms of the target trials with SSNA bursts showed larger positive deflection in the early part of the P300 component than those of the target trials without SSNA bursts. We conclude that SSNA is generated in relation to the conscious cognitive process, as well as to the automatic process activated by changes in repeating stimuli. The early part of the P300 component (possibly P3a) may be related to the mechanisms that generate SSNA.