The family of guanylin regulatory peptides, including guanylin and uroguanylin, are strongly expressed in intestinal mucosa and regulate intestinal fluid secretion during digestion. Guanylins are also involved in acid neutralization and the regulation of membrane-bound guanylate cyclase signaling molecules. Guanylin and uroguanylin are secreted primarily in the stomach, intestine, and colon. Uroguanylin is an endogenous activator of intestinal guanylate cyclase. It is a paracrine and/or autocrine regulator of intestinal water and salt transport. Uroguanylin stimulates intestinal guanylate cyclase through the same receptor binding region as the heat-stable enterotoxins. Uroguanylin is involved in the regulation of intestinal fluid and electrolyte transport.