Several series of dihydrostilbenamide, imidazo[2,1-a]isoindole, pyrimido[2,1-a]isoindole and phthalazinone derivatives were obtained and their vasorelaxant activity was measured on isolated rat aorta rings pre-contracted with phenylephrine (10(-5)M). Some phthalazinones attained, practically, the total relaxation of the organ at micromolar concentrations. For the most potent compound 9h (EC(50)=0.43microM) the affinities for alpha(1A), alpha(1B) and alpha(1D) adrenergic sub-receptors were determined.