Several a-adrenoreceptor antagonists were prepared by coupling one of the two moieties of WB 4101 (1) with oneof the two moieties of prazosin (2). Their blocking activity and relative selectivity on al- and a,-adrenoreceptorswere evaluated in the isolated rat vas deferens. Although retaining a significant selectivity toward al-adrenoreceptors,all the drugs were weaker antagonists than the parent compounds 1 and 2. Opening the piperazine ring of 2 gave3, which displayed a very high activity and selectivity toward a,-adrenoreceptors (a1/a2= 3890). This may haverelevance in understanding the mode of action of prazosin. In addition, 3 may represent a valuable tool in thecharacterization of a-adrenoreceptor subtypes.