The effect of the duodenal ulcerogen cysteamine on secretion of epidermal growth factor from Brunner's gland pouches was studied in the rat. Total output of immunoreactive epidermal growth factor was reduced to approximately 55%, compared with controls, 5 h after administration of cysteamine (300 mg/kg, s.c.). Furthermore, measurements on tissue extracts of the pouches revealed that 5 h after cysteamine treatment, Brunner's glands were depleted of epidermal growth factor. The effect on ulcer development of intraduodenally applied exogenous epidermal growth factor (1 micrograms/kg . h) also was studied. Luminal epidermal growth factor significantly inhibited the formation of cysteamine-induced duodenal ulcer, compared with controls receiving saline. The effect was not due to inhibition of gastric acid secretion or stimulation of duodenal bicarbonate secretion since the dose of epidermal growth factor used, when tested on chronic fistula rats, had no effect on acid secretion and did not influence bicarbonate secretion from Brunner's gland pouches. These results demonstrate that epidermal growth factor has a cytoprotective effect on the duodenal mucosa, and it is suggested that inhibition of synthesis and secretion of endogenous epidermal growth factor may be a pathogenetic factor in cysteamine-induced duodenal ulcer.