A series of 5-arylidenepyridazin-3-ones substituted in the 2-position by an arylpiperazinoalkyl moiety (2-16) was synthesized and evaluated for analgesic activity. In the phenylbenzoquinone-induced writhing test, Mannichbases 2-14 were the most active compounds (6.1 < or = ED50 < or = 43.0 mg/kg, orally; ED50 is the half-maximal effective dose). Pyridazinones 8 and 9, with a 3-chlorophenylpiperazinomethyl