We studied the effects of electroacupuncture at Zusanli (ST 36) on neurons in the colonic myenteric plexus and on defecation in rats with irritable bowel syndrome with constipation (IBS-C). We also used intragastric administration of pinaverium bromide as a positive control treatment to reveal the pathway mediating the onset of IBS-C. Both electroacupuncture and pinaverium bromide greatly improved defecation in rats with IBS-C. Immunohistochemical staining of the enteric nervous system neuronal marker protein gene product 9.5 in the colonic myenteric plexus showed that electroacupuncture by itself, or in combination with pinaverium bromide, increased the number of neurons and the staining intensity of protein gene product 9.5 in the colonic myenteric plexus. We conclude that visceral hypersensitivity is likely to be a primary cause of constipation in IBS-C rats.