Abstract: In order to develop more potent and less toxic, antithrombotic agents, ten 6-(4-substituted piperazinyl acetyl aminophenyi)-4, 5-dihydro-3(2H)-pyridazinones were synthesized. The title compounds were tested in vitro for platelet aggregation inhibitory activity with ADP-induced rat platelets and PAF-induced rabbit platelets. Preliminary tests showed that all of the pyridazinones could inhibit ADP-induced rat platelet aggregation. Ⅰ₇, Ⅰ₈, Ⅰ₉ were more potent than the control compound CI 930. Ⅰ₉ was the most potent compound with IC₅₀ of 0.99 μmol/L. Pertaining to PAF-induced rabbit platelet aggregation. Ⅰ₉ was the most potent inhibitor with IC₅₀ of 3.7 μmol/L.