A series of pyridazinone-containing compounds were designed and synthesized as congeners for diclofenac, the most potent and widely used NSAID. The target compounds were evaluated for their anti-inflammatory activity on rat paw edema inflammation model against diclofenac as a reference compound. Seven of the tested compounds demonstrated more than 50% inhibition of carrageenan-induced rat paw edema at a dose 10 mg/kg. The compounds, 6-(2-bromophenylamino)pyridazin-3(2H)-one 2a and 6-(2,6-dimethylphenylamino)pyridazin-3(2H)-one 2e, displayed 74 and 73.5% inflammation inhibitory activity, respectively, which is comparable to diclofenac (78.3%) at the same dose level after 4 h. The most active compounds as anti-inflammatory agents, 2a, 2e, and 6a, displayed fewer number of ulcers and milder ulcer score than indomethacin in ulcerogenicity screening.