Abstract: In searching for effective anticonvnlsant agent fourteen 6- (substitutedphenyl )-4, 5- dihydro- 3 (2H) pyridazinones and fifteen 6- (substitutedphenyl)-3 (2H) pyridazinones have been synthesized and screened in mice for their ability to antagonize maximal electroshock seizure (MES). The ED₅₀ values showed that 6- (2 ', 4 '-dichlorophenyl )-3 (2H) pyridazinone was the most potent anticonvulsant in these compounds. The structure- activity relationships in these two series were examined. We came to the conclusion that the higher is the hydrophobic parameter π of the substituent on phenyl ring, the more potent anticonvulsant is the compound. And also, only the compounds with an electronwithdrawing substituent on the phenyl ring exhibited appreciable anticonvulsant activity.