Selectivity of tamsulosin and terazosin to functional α 1-adrenoceptors was examined. Both drugs competitively inhibited the contractile responses to noradrenaline in different tissues where the responses were mediated through the α 1D-, α 1B-or α 1L-subtype. Together with the affinities obtained in the binding study with cloned (α 1a, α 1b, α 1d) and native (α 1A and α 1B) subtypes, the selectivity of tamsulosin was α 1A> α 1L, α 1D> α 1B. Terazosin had lower affinity at various subtypes than tamsulosin, but showed relatively high selectivity to the α 1D-subtype. In the human prostate, tamsulosin was more than 30-fold higher in affinity than terazosin in functional and binding studies.