The participation of polyamines and nonprotein sulfhydryls in the gastric cytoprotective mechanisms was studied using gastric mucosal lesions produced by acidified ethanol in rats as an experimental model. Treatment with prostaglandin E2 (PGE2), but not cimetidine, prevented the formation of gastric mucosal lesions. Oral administration of cadaverine, spermidine and spermine prevented the lesion formation by acidified ethanol in a dose-dependent manner. Indomethacin or acetazolamide had no influence on the cytoprotective effect of spermine, whereas sulfhydryl blockers such as iodoacetamide and N-ethylmaleimide partially blocked it. Sulfhydryl compounds such as cysteine, reduced glutathione (GSH), and cysteamine prevented the lesion formation induced by acidified ethanol. The concentration of nonprotein sulfhydryls in the gastric mucosa was significantly decreased at 1 hr after administration of acidified ethanol, and this decrease was partially prevented by spermine or PGE2. These results suggest that the cytoprotective effect of spermine may not be mediated by endogenous prostaglandins or alkaline secretion in the gastric mucosa, but may be partially related to endogenous sulfhydryl compounds.