Nineteen new [6-(4-methoxyphenyl)-3(2H)-pyridazinone-2-yl]acetamide (1-10) and 3-[6-(4-methoxyphenyl)-3(2H)-pyridazinone-2-yl]propanamide (11-19) derivatives have been synthesized in this study. The structures of the compounds have been elucidated by their IR and NMR spectral data and elemental analysis. Anti-nociceptive activity of the compounds has been investigated by modified Koster's Test in mice, using aspirin as a reference. All the compounds (at 100 mg/kg dose) except 1 and 9 have been found more potent than aspirin. Compound 6 in the group of : acetamide derivatives and compound 15 in the group of propanamides exhibited the highest antinociceptive activity. In addition, the propanamides have generally been found more potent than acetamides. In addition to these studies, the quantitative relationships between some structural parameters (such as log P, parachor, molar refractivity, and molecular connectivity indices) and antinociceptive activity of the compounds have been investigated. Statistical regression analysis has shown a close relationship to exist between the first-order molecular connectivity index ((1)chi) and the antinociceptive activity.