Seventy-three women with posttraumatic stress disorder (PTSD) resulting from rape or physical assault participated in a loud-tone procedure, while skin conductance (SC), heart rate, and electromyogram responses were recorded. Pearson correlations were examined between each psychophysiological response and Clinician-Administered PTSD Scale (CAPS) symptom scores. Significant correlations were adjusted for each remaining individual PTSD symptom score. Heart rate response (HRR) significantly correlated with CAPS total score and with CAPS nightmares. The relationship between HRR and nightmares remained significant after controlling for each of the other 16 individual PTSD symptoms, for the remaining reexperiencing cluster, and for CAPS total score. The zero-order correlations between SC response and nightmares and between electromyography response and nightmares were both not significant. The association of nightmares with larger HRR in the absence of an association with larger SC response likely reflects reduced parasympathetic tone. Thus, our findings indirectly support a role for reduced parasympathetic tone in PTSD nightmares.