Fused pyridazine derivatives represented by formula (I) or pharmaceutically acceptable salts thereof (wherein each symbol has the meaning as defined in the specification.). ##STR00001## Because of inhibiting poly(ADP-ribose)polymerase, the compounds represented by formula (I) are useful as preventives and/or remedies for various ischemic diseases (in brain, cord, heart, digestive tract, skeletal muscle, retina, etc.), inflammatory diseases (inflammatory bowel disease, multiple cerebrosclerosis, arthritis, etc.), neurodegenerative diseases (extrapyramidal disorder, Alzheimer's disease, muscular dystrophy, lumbar spinal canal stenosis, etc.), diabetes, shock, head trauma, renal failure, hyperalgesia, etc. Moreover, these compounds are useful as agents against retroviruses (HIV etc.), sensitizers in treating cancer and immunosuppressants.