The ulcerogenic activity of a 3% ketoprofen gel (ID-GEL) after its transdermal application in rats was determined with the Litchfield and Wilcoxon method in reference to the oral administration of ketoprofen in a suspension. The $UD_{50}$ (dose producing ulcers in 50%, of the rats tested) of ID-GEL after its transdermal application was approximately 4 times greater than that after the oral administration of the drug, indicating that the ulcerogenic activity of ketoprofen was much lowered with its transdermal application.