We studied the effects of a mild irritant, 10% ethanol, and sofalcone on the gastric mucosal defense mechanisms in newborn rats, comparing the effects to those seen in adult rats. The results indicated (1) that both mucosal prostaglandin E2 (PGE2) content and mucus gel layer thickness increased with age, (2) that sofalcone, but not 10% ethanol, increased mucosal PGE2 content and mucus gel thickness in 1- and 8-week-old rats, (3) that both sofalcone and 10% ethanol decreased mucosal damage induced by 50 or 75% ethanol in both age groups, and (4) that 10% ethanol, but not sofalcone, decreased ethanol-induced mucosal damage in rats pretreated with indomethacin. We concluded that 10% ethanol and sofalcone increase gastric mucosal defense mechanisms in newborn rats as in older rats.