The effect of acute and chronic ASA administration on stomach and duodenal mucosa was studied on white male Wistar rats. ASA (250 mg/kg b.w.) was administered by esophageal intubation in a single dose, and the rats were sacrificed at 2, 6, 24 hours, or after two, three, ten and twenty days, 24 hours after the last ASA application. Two types of erosions were found in the first 24 hours: superficial and deep. Superficial erosions showed advanced healing by the 24th hour. Deep erosions appearing between 6 and 24 hours from the beginning of experiment became deeper, after two and three days ASA application. They had morphological features of chronic ones by the 10-20th day. ASA resorbtion was most expressed by the 3rd day from the experimental onset (plasma concentration 0.348 +/- 0.052 g/l). The resorbtion was decreased in chronic experiment, and plasma concentration (0.236 +/- 0.046 g/l, 0.244 +/- 0.045 g/l) showed statistically reliable lower values.