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Abstract: A series of 4-amino-5-vinyl-3(2H)-pyridazinones and analogues were synthesized and their antinociceptive effect was evaluated in the mouse abdominal constriction model. Several of the novel compounds showed ED50 values in the range 6–20mg/kg/sc and demonstrated to be able to completely protect all the treated animals from the effect of the noxious stimulus at 30mg/kg/sc. SAR studies confirmed the essential role played by an amino or substituted amino function at position 4 and by a vinyl group at position 5 of the diazine system. [Copyright &y& Elsevier]