This pooled analysis of data from two randomized, placebo-controlled trials of venlafaxine extended release (ER) assessed onset of activity and time to response on the 17 symptoms of post-traumatic stress disorder (PTSD) listed in DSM-IV and measured by the 17-item Clinician-Administered PTSD Scale (CAPS-SX17). The intent-to-treat (ITT) population comprised 687 patients (placebo, n=347; venlafaxine ER, n=340). Significant (p<0.05) separation between venlafaxine ER and placebo was observed on most CAPS-SX17 items, with earliest onset of activity and response (week 2) on items 5 (physiological reactivity on exposure to cues) and 14 (irritability or anger outbursts), and (week 4) items 1 (intrusive recollections) and 4 (psychological distress at exposure to cues). Onset of activity and response occurred later (generally, weeks 6–8) on items 9 (diminished interest/participation in activities), 10 (detachment or estrangement), 11 (restricted range of affect), 12 (sense of foreshortened future), all associated with numbing, 15 (difficulty concentrating), 16 (hypervigilance), 17 (exaggerated startle response), associated with hyperarousal, and 6 (avoidance of thoughts/feelings or conversations). Significant differences between venlafaxine ER and placebo were largely absent throughout the treatment period and at the primary week-12 end-point for items 2 (distressing dreams), 7 (avoidance of activities, places or people), 8 (inability to recall important aspect of trauma) and 13 (difficulty falling/staying asleep). These results indicate that symptoms of physiological reactivity and psychological distress in response to cues, and irritability/anger outbursts show early and robust improvement with venlafaxine ER treatment, while symptoms of numbing and hyperarousal take longer. The early and persistent effect of venlafaxine ER over placebo on anger/irritability is noteworthy in view of the clinical significance of these symptoms in PTSD.