The present study examined how cold pressor stimulation influences electrophysiological correlates of arousal. We measured the P50 auditory evoked response potential in two groups of subjects who immersed their foot in either cold (0-2 degreees Celsius) or room temperature (22-24 degreees Celsius) water for 50 seconds. The P50, which was recorded before and after stimulation, is sleep-state dependent and sensitive to states of arousal in clinical populations. We found a significant reduction in P50 amplitude after exposure to cold, but not room temperature water. In comparison with other studies, these results indicate that cold pressor stimulation in normal subjects may evoke a regulatory process that modulates the P50 amplitude, perhaps to preserve the integrity of sensory perception, even as autonomic and subjective aspects of arousal increase.