This study was designed to determine the gastroprotective actions of an antacid (Maalox 70) and its components, Al(OH)3 and Mg(OH)2, against acute gastric lesions induced by absolute ethanol, acidified aspirin (ASA), and water immersion and restraint stress in rats. Given orally, the antacid prevented dose-dependently the formation of gastric lesions by all three ulcerogens, and these effects were similar to those obtained with a methylated prostaglandin E2 (PGE2) analog. Active Al(OH)3 gel was equipotent with Maalox, whereas Mg(OH)2 was significantly less effective in gastroprotection than Maalox 70. Chemically inactive Al(OH)3 wet gel showed only small and insignificant protective properties. Since the gastroprotective activities of Maalox 70 against ethanol lesions cannot be reversed by pretreatment with indomethacin, and since neither Maalox 70 nor its active components affected the mucosal generation of PGE2 and leukotriene C4, we postulate that mucosal prostanoids are not the primary mediators in the mechanism of their protective action on the gastric mucosa.