Herein, we report the synthesis and pharmacological properties of several series of pyridazine and pyridazinone derivatives. All the synthesized compounds were tested, in vivo, for their anti-inflammatory and ulcerogenic properties against indomethacin, as a reference compound. Compounds 4a and 9d have shown a potent anti-inflammatory activity more than indomethacin with rapid onset of action and safe gastric profile. The latter compounds were then selected for further investigation. In the MTT assay in vitro, both compounds were identified as potent and selective COX-2 inhibitors.