Aims: Our main aim was to compare the prostatic selectivity of silodosin with that of other a 1 -adrenoceptor (AR) antagonists. Methods: We examined uroselectivities in two sets of experiments namely, in vitro and in vivo functional studies using male dogs. In the in vitro study, after evaluating the inhibitory e¡ects of silodosin on noradrenaline (NA)-induced contractions in the isolated prostate and isolated carotid artery using the Magnus method, we calculated prostatic selectivity. In the in vivo study, we examined the e¡ects of drugs on the hypogastric nerve stimulation (HNS)-induced increase in intraurethral pressure (IUP) and on blood pressure. The uroselectivity of silodosin was compared with those of tamsulosin and naftopidil. Results: In vitro, all drugs antagonized NA-induced contraction in both prostate and carotid artery. The prostatic selectivity of silodosin (79.4) was much higher than those of tamsulosin (1.78), naftopidil (0.55), BMY 7378 (0.115), and prazosin (0.01). In vivo, intravenously (i.v.) administered silodosin dose-dependently inhibited the HNS-induced increase in IUP with much less hypotensive e¡ect than either tamsulosin or naftopidil, the uroselectivity (ED 15 /ID 50 ) of silodosin (237) being signi¢cantly higher than those of tamsulosin (1.21) and naftopidil (2.65). Conclusions: Our results clearly demonstrate that silodosin is a potent and highly selective a 1A -AR antagonist. A selective a 1A -AR antagonist such as silodosin may have good potential as a less-hypotensive drug for the treatment of urinary dysfunction in benign prostatic hyperplasia patients.