Heterocyclic derivatives of the formula (I) 1 wherein each symbol is as defined in the specification, pharmaceutically acceptable salts thereof and method of producing them. Pharmaceutical compositions containing the heterocyclic derivative or a pharmaceutically acceptable salt thereof, particularly, ACAT inhibitors and lipoperoxidation inhibitors. The heterocyclic derivatives and pharmaceutically acceptable salts thereof of the present invention show superior ACAT inhibitory activity and lipoperoxidation inhibitory activity, and are useful as ACAT inhibitors and hyperlipemia inhibitors. To be specific, they are useful for the prevention and treatment of arteriosclerotic lesions of arteriosclerosis, hyperlipemia and diabetes, as well as ischemic diseases of brain, heart and the like.