A new patented chemical agent (Al-Mg-hydroxy-carbonate; acid-binding capacity greater than 30 mmol/g) was produced by our work-team. After our preliminary pharmacological and some prospective, randomized, multicentre, controlled clinical studies, this antacid was registered (Tisacid tablet and suspension; Alkaloida, Hungary). A cumulative ulcer healing rate of 80-85% was proved by Tisacid monotherapy applied in low doses (from 80 to 160 mmol/day) in patients with duodenal ulcer. The aims of this study were: (i) to determine the role of different antacids on the genesis of mucosal prostaglandins (PGs) (PGE2 and 6-keto-PGF1 alpha) in normal rats; (ii) to evaluate the effects of indomethacin pre-treatment (20 mg/kg b.w.,s.c.) on the Tisacid-induced alterations of gastric mucosal PG-contents; (iii) to analyse the generation of oxygen free radicals and lipid peroxidation in the rat oxyntic mucosa by the application of different doses of Tisacid (activities of CAT, GSH-px and SOD, contents of MDA and red. GSH). It was found that: Tisacid has a potent gastroproprotective effect in gastric mucosa, via (a) an increase in the mucosal levels of PGs, and (b) a scavenging-like effect in normal rat gastric mucosa. It is concluded that the gastroprotective effect of Tisacid appears because of the following: (i) excellent acid-neutralizing capacity; (ii) mucosal generation of PGs (PGE2 and PGI2); (iii) free radical scavenging; (iv) its possible activity as a Ca-antagonist (Mg-containing compound).