Recent evidence indicates that human urogastrone-epidermal growth factor originates in submandibular and Brunner's glands and that serum levels are low (less than 1 ng ml-1). It occurs in many secreted fluids to a much greater extent and many tissues are thus exposed to concentrations greater than 100 ng ml-1. Rapid actions in vivo of URO-EGF include the ability to inhibit gastric acid secretion at low doses (250 ng kg h-1 in humans) and to provide cytoprotective effects against ulcerogenic agents (250 ng kg h-1 in cats). More prolonged exposure of tissues shows increases in parameters related to wound healing, beneficial effects upon ulceration and also the ability to accelerate crypt cell production rate along the gastrointestinal tract. Synthetic material has been prepared with an identical structure to the natural URO-EGF thus enabling detailed studies of the biological effects to be pursued.