The effect of mebendazole on the gastric and intestinal mucosa was studied in 52 intact white rats. It was shown that the drug caused a significant damage to the mucosa, especially of goblet cells of villi and crypts and interfere with the mucoid secretion. The alterations after mebendazole administration were more severe than those caused by naphthamon. On the 13-15th day after the treatment the complete restoration of the damage was not seen.