Abstract A series of di-and tri-substituted 6-phenyl-4, 5-dihydro-2H-pyridazine-3-ones is described. The compounds were designed to be antithrombotics and were assessed for their inhibitory properties on platelet aggregation and on thrombus formation in an arteriovenous shunt in the rat. The synthesis and physical properties of the compounds are described. The structure-activity relationships reveal that non-aromatic nitrogen- ...