We measured event-related brain potential (ERP) component amplitudes and heart rate (HR) to four intensities of randomly presented tones in two matched groups of drug-free male Vietnam veterans: 12 patients diagnosed with posttraumatic stress disorder (PTSD) and 6 normal combat veterans. Subjects were evaluated with structured diagnostic interviews and anxiety and depression rating scales. We found a significant group X intensity interaction for P2 peak amplitude at CZ. Subjects were classified as augmenters or reducers: positive P2 slopes as a function of stimulus intensity implying augmentation and negative slopes implying reduction. Nine of 12 PTSD subjects were reducers (sensitivity of 75%) and 5 of 6 normals were augmenters (specificity of 83.3%). By the third and fourth second following tone onset, the mean HR of PTSD subjects increased more than twice that of the normals. HR change scores were significantly responsive to the manipulation of stimulus intensity and to the difference between our two groups. P2 reduction differentiates Vietnam veterans with combat-related PTSD from combat veteran controls, and PTSD subjects are more autonomically arousable than their combat veteran peers.