This study will employ an intent-to-treat design to evaluate the relationship between exercise-induced posttraumatic stress disorder (PTSD) symptom reduction and exercise-induced changes in cortisol level. The participants consist of nurses and the intervention will be conducted at the Clinical and Translational Science Center of the University of New Mexico. The study was approved by the Human Research Protections Office of the university. Nurses who are screened positively for PTSD will be randomly assigned to either control or exercise group. At baseline and in weeks 4, 8, 12 and 16, immediately after the phlebotomy for serum cortisol, the investigators will ask the participants to rate the severity of their PTSD symptoms using the PTSD Checklist (PCL).