Indomethacin induces inflammation of the small-intestinal mucosa leading to ulceration in patients. In rats we examined this untoward drug effect by measuring changes inde novo macromolecular synthesis and morphology during exposure to indomethacin given as a single oral dose of 15 mg/kg. Indomethacin alone induced diffuse jejunoileal mucosal inflammation accompanied by spotty ulceration. These lesions were not observed at 2 and 6 hr after administering the drug but were apparent by 24 hr. By 72 hr the intestinal inflammation had intensified, and there were multiple perforating ulcers, extensive adhesions, and peritonitis in the moribund animals.De novo DNA synthesis was increased approximately fourfold by 24 hr and ninefold by 72 hr; RNA synthesis was nearly doubled by 24 hr and nearly trebled by 72 hr; protein synthesis increased about fourfold at 24 hr and eightfold at 72 hr. Both the histological lesions and the macromolecular synthetic changes induced by indomethacin were prevented by 16,16-dimethyl prostaglandin E2 and by broad-spectrum antibiotics but not by dexamethasone, prednisolone, or colchicine. Neither ligation of a mesenteric arterial branch nor administration of vasopressin induced the intestinal inflammatory lesions seen with indomethacin. The predominant mechanism involved in indomethacin ulceration of the gut appears to be acute inflammation rather than ischemia.