The solid-phase parallel preparation of a library of 4,5-dihydropyridazin-3(2H)-one derivatives substituted at position 6 with piperazinylmethyl or tetrahydroquinolinylmethyl groups and analogues (3) is reported. Polymer-supported gamma-keto-delta-aminoesters prepared from Wang resin reacted with hydrazine or methylhydrazine to afford pyridazinones in good yields after a cyclization cleavage approach. We have evaluated these novel analogues and several compounds of other series (1, 2) for their vasorelaxant effect. Among the products tested, 3l and 3d proved to be efficacious and potent relaxant agents of the isolated rat aorta. Inhibitors of phosphodiesterase (PDE3), responsible for the breakdown of cyclic AMP in the vascular smooth muscle, are currently developed for cardiac heart failure because of their inotropic effect and coronary vasodilatation. We had expected that the vasodilatation induced by 3l, as efficient as reference PDE3 inhibitors, milrinone or CI-930, to be due to PDE3 inhibition. However 3l and 3d exhibited a low inhibitory effect against PDE3 isoenzyme activity. These compounds induced a significant vasorelaxation, which could be of therapeutic interest even if their mechanism of action remains to be determined.