We studied the effects on gastric fundic and antral epithelial proliferation of acute water immersion restraint stress in rats and of chronic indomethacin ingestion in rats and humans using autoradiographic methods. Acute stress appeared to inhibit fundic epithelial proliferation but had no effect on antral proliferation in rats. We conjecture that this inhibition of fundic epithelial proliferation may help explain the development of stress-related mucosal lesions, which are more likely to occur in fundic mucosa. Chronic indomethacin ingestion stimulated fundic epithelial proliferation but had no effect on antral proliferation in both rats and humans. From these observations we conjecture that the failure of antral epithelial proliferation to respond to indomethacin may account in part for the ulcerogenic action of this drug.