Substituted 5-hydrazinopyridazin-3(2H)-ones, e.g., 4-chloro-2-(3-trifluoromethylphenyl)-5-(1-methylhydrazino)-pyridazin-3(2H)-one or 4-chloro-2-(2-fluoro-5-trifluoromethylphenyl)-5-(1-methylhydrazino)-pyridazin-3(2H)-one, are prepared by treating halogena