Nitrogen monoxide synthase NOS inhibitors comprising as the active ingredient 2-aminothiazoline or oxazoline derivatives represented by the following general formula I or physiologically acceptable acid addition salts thereof: I wherein R1 and R2 are the same or different and each represents hydrogen or lower alkyl X represents oxygen or sulfur, provided that R1 and R2 represent hydrogen atoms at the same time and each ast stands for an asymmetric carbon atom in case where R1 or R2 is not hydrogen. These compounds have an extremely high inhibitory activity on iNOS and have a high selectivity, which makes them useful as NOS inhibitors with little side effects.
