Numerous papers have emphasized the damaging effect of salicylic acid compounds on the stomach mucosa. The investigations here reported aimed at examining the effect on gastric and duodenal morphology of a moderate dose of aspirin. Oxidative enzymes were evaluated histochemically. To rats in groups of 16, aspirin tablets, placebo tablets, or the same with additional 1 cc of 0.1 N HCl, were administered twice a day for 6 weeks. Approximately 160 mg/kg/24 hrs. were ingested. Microscopy revealed severe hyperaemia along with focal gastritis in the aspirin treated animals. Chronic gactric ulcers were observed in 18 rats in the aspirin treated groups, particularly in the group where HCl was added. Ulcerations in the area of the pyloricgland occurred in the latter group only. A reduction in mucopolysaccharides was demonstrated and involved the acid as well as the neutral ones. NADH‐reductase and cytochrome oxidase were inhibited not only in the surface cells but also in those in the deeper layers. The reduction in oxidative enzymes in otherwise undamaged areas suggests an interference with the cellular metabolism, probably processes in the respiratory chain which might lead to reduction in energy‐rich phosphate bo