The present invention features peptides, compositions, and related methods for treating gastrointestinal disorders and conditions, including but not limited to, irritable bowel syndrome (IBS), gastrointestinal motility disorders, functional gastrointestinal disorders, gastroesophageal reflux disease (GERD), duodenogastric reflux, Crohn's disease, ulcerative colitis, inflammatory bowel disease, functional heartburn, dyspepsia, visceral pain, gastroparesis, chronic intestinal pseudo-obstruction (or colonic pseudo-obstruction), disorders and conditions associated with constipation, and other conditions and disorders are described herein, using peptides and other agents that activate the guanylate cyclase C (GC-C) receptor.