We have synthetised a series of oxidised apomorphine derivatives (orto and para quinones 2-5), in order to analyse their vascular activity. We have performed radioligand binding assays on rat cortical membranes and functional studies on rat aortic rings. Instead the relaxant activity exhibited by (R)-apomorphine, o-quinones 2, 4, show contractile activity dependent on endothelium in rat aortic rings. Compound 2, the main metabolite of (R)-apomorphine auto-oxidation, was the product which showed enhanced contractile activity by a complex mechanism related to activation of Ca(2+) channels through release and/or inhibition of endothelial factors. Moreover, this compound disrupts the endothelial function as shows the lack of response to acetylcholine observed in vessels pretreated with it.