Background: Stress is a major contributing factor to the pathophysiology of IBS and accelerates colonic transit and increases in defecation. The parasympathetic vagal and pelvic nerves regulate colonic motility and defecation in physiological state. It was shown that physical stress altered colonic functions via vagal efferent in rats. However, it remains still unclear whether the pelvic efferent pathway is involved in accelerating colonic and rectal functions induced by stress. The aim of this study was to investigate whether distal colonic transit and defecation are accelerated via pelvic efferent nerve in psychological stress-loaded rats. We also evaluated the effect of HPA-axis activation on stress -induced defecation. Method: The rats were subjected to water avoidance stress (WAS) or sham stress for 1-hr or 2-hr. The number of fecal pellets was counted for 1-hr in WASor sham stress-rats. Adrenalectomy (Adx), subdiaphragmatic vagotomy (Vag), pelvic nerve rectal branches (Rectal nerves) transection (RNT), or each sham operation was performed at 1 week before WAS. Colonic transit was assessed by the distribution of phenol red injected from catheter located in proximal colon. The distal part of colonic and rectal transit was measured over a 2-hr WAS period from the evacuation rate of a bead inserted into distal colon. c-Fos immunoreactivity (IR), used as a marker of neuronal activation, was monitored in longitudinal muscle/myenteric plexus (LMMP) whole mount preparations of proximal and distal colon after WAS and sham stress. Results: WAS significantly accelerated colorectal transit and defecation with a marked elevation of plasma corticosterone (CORT) levels. Increased defecation induced by WAS was significantly reduced by Vag and RNT, even though CORT levels were elevated. WASinduced acceleration of transit in distal colon and rectum was significantly reduced by RNT, compared with sham-operation. The number of c-Fos-IR cells were significantly increased by WAS in LMMP of proximal and distal colon. In RNTrats, an increase in c-Fos IR cells after WAS was decreased in distal colon, but not in proximal colon, whereas Vag did not affect an increase of c-Fos IR cells in distal colon. In contrast, in Adx-rats WAS increased the number of defecation without an elevation of plasma CORT. Conclusion: It is concluded that psychological stress accelerated distal colonic transit and rectal transit via a parasympathetic efferent pelvic pathway. It seems that the stimulation of pelvic nerve in response to stress activates myenteric neurons in distal colon, followed by an increase of colonic motility. On the other hand, a vagal efferent pathway affects proximal colonic transit. However, an increase in defecation induced by acute psychological stress might not involve the activation of the HPA axis.