Background: Irritable bowel syndrome (IBS) causes chronic abdominal pain and abnormal bowel movements. The etiology involves complicated interactions among visceral hypersensitivity, disorders related to bowel movements, and stress. Changes in the microbiota affect the IBS pathophysiology. We investigated changes in colorectal motility, structure, and microbiota in response to stress due to maternal separation (MS) and corticotropin-releasing hormone (CRH) administration in rats. Summary: Neonatal rats were separated from their mothers for 3 h daily from postnatal day (PND) 2 to PND14. The control group included rats of the same age that were not separated. After MS, the rats were undisturbed for 5 weeks. At 8 weeks of age, 10 mu g of CRH or saline was intravenously administered to MS and control groups. Two hours later, the number of fecal pellets was counted. Three hours after CRH or saline administration, the rats were sacrificed and colorectal tissue samples and cecal contents were collected to analyze the fecal microbiota. The number of fecal pellets was significantly greater in MS with the CRH group. Both stressors altered the microbiota composition. Key Messages: Among rats that received CRH, MS increased the colorectal motility. Stress due to MS altered the gut microbiota composition. (C) 2016 S. Karger AG, Basel.