Comparisons were made between a group of male Vietnam veterans suffering from Post-Traumatic Stress Disorder (PTSD) (n = 11) and an age and sex matched group of non-veteran controls (n = 11) on their psychophysiological responding (heart rate (HR), blood pressure (BP), forehead EMG, skin resistance level, and peripheral temperature) to mental arithmetic and an audiotape of combat sounds played at gradually increasing volume levels. The two groups responded differently to the combat sounds in terms of HR, systolic BP, and forehead EMG. The HR response could correctly classify 95.5% of the combined sample. Implications of these findings for the basis of PTSD are discussed.