Comparative studies were made on some sulfonamides, used individually and combined with parasympatholytic agents as regards (1) the absorption rate through isolated rat small intestine (in vitro), (2) the absorption rate through rat small intestine (in vivo), and (3) the blood concentration of sulfonamides were examined by its oral administration with each combined drug to rabbits, and the following effects were found, parasympatholytic agents inhibit the absorption of sulfonamides from the small intestinal tract. Comparison of the inhibitic efficiency of parasympatholytic agents is as follows: oxazepam, oxyphencyclimine hydrochloride, probantheline bromide, atropine sulfate (The examples are from the weakest to the strongest). Decrement of the absorption rate of sulfonamides is as follows: sulfadiazine, sulfathiazole, sulfamethoxypyridazine, sulfadimethoxine, sulfamerazine, sulfamethazine (The example are from the strongest to the weakest). Additionally, it is assumed that if the combined drugs were absorbed from the small intestine in the original form, those inhibitic effects should be best regarding to their sulfonamide per parasympatholytic agent combined rate "25:1" than to their any other rates.her rates.