The effect of oral administration of synthetic human epidermal growth factor/urogastrone (EGF/URO) on healing of chronic duodenal ulcers induced by cysteamine in rats was investigated and compared with that of cimetidine, a H2-receptor antagonist. After 25 and 50 days of treatment, synthetic human EGF/URO significantly increased healing of chronic duodenal ulcers to the same extent as cimetidine. Combined treatment with synthetic human EGF/URO and cimetidine for 25 days was more effective than synthetic human EGF/URO given alone, whereas combined treatment for 50 days was significantly more effective than cimetidine alone. These results show that a combination of an agent inhibiting gastric acid secretion and the cytoprotective and growth-stimulating peptide EGF/URO seems to be more effective with regard to duodenal ulcer healing than individual administration of the two substances. Synthetic human EGF/URO is a potent inhibitor of gastric acid secretion when administered intravenously, but had no effect on acid secretion when given intraduodenally, which suggests that the effect of synthetic human EGF/URO is a direct action on the duodenal mucosa. In conclusion, this study showed that oral synthetic human EGF/URO has a significant effect on healing of duodenal ulcers in rats. The amount of synthetic human EGF/URO administered is comparable to that found in saliva during stimulation of the salivary glands. Our results, therefore, suggest that EGF/URO is one of the endogenous factors participating in healing of duodenal ulcers.