1(2H)-Phthalazinones such as trans-6-[2-(4-methoxyphenyl)ethenyl]-2-(3-aminopropyl)phthalazin-1(2H)-one hydrobromide, trans-6-[2-(4-methoxyphenyl)ethenyl]-2-[3-(dimethylamino)propyl]phthalazin-1(2H)-one hydrobromide, and trans-6-[2-(4-hydroxphenyl)ethenyl]-2-[3-(dimethylamino)propyl]phthalazin-1(2H)-one hydrochloride are useful as cytoprotective agents in mammals.