Three novel series of 2-(substituted phenyl)-4-(substituted arylidene)-imidazolone-5-(4H)-ones were derived from the corresponding oxazolones by condensation with different arylamines. Eleven of the synthesized compounds were selected and evaluated for their effect on carrageenan-induced rat paw edema. Compound 4b had the same efficacy as the reference standard (indomethacin), and compounds 3b, 3c, 4a, 4d and 9a showed good to excellent activities, with other compounds only weakly active. The potent compounds were evaluated for their inhibitory activities against COX-2-catalyzed $PGE_2$ production, with 4a, 4b and 3c showing strong inhibitory activity.