It has been hypothesized that in patients with benign prostatic hyperplasia, selective antagonism of the alpha1A-adrenoceptor-mediated contraction of lower urinary tract tissues may, via a selective relief of outlet obstruction, lead to an improvement in symptoms. The present study describes the alpha1-adrenoceptor (alpha1-AR) subtype selectivities of two novel alpha1-AR antagonists, Ro 70-0004 (aka RS-100975) and a structurally-related compound RS-100329, and compares them with those of prazosin and tamsulosin. Radioligand binding and second-messenger studies in intact CHO-K1 cells expressing human cloned alpha1A-, alpha1B- and alpha1D-AR showed nanomolar affinity and significant alpha1A-AR subtype selectivity for both Ro 70-0004 (pKi 8.9: 60 and 50 fold selectivity) and RS-100329 (pKi 9.6: 126 and 50 fold selectivity) over the alpha1B- and alpha1D-AR subtypes respectively. In contrast, prazosin and tamsulosin showed little subtype selectivity. Noradrenaline-induced contractions of human lower urinary tract (LUT) tissues or rabbit bladder neck were competitively antagonized by Ro 70-0004 (pA2 8.8 and 8.9), RS-100329 (pA2 9.2 and 9.2), tamsulosin (pA2 10.4 and 9.8) and prazosin (pA2 8.7 and 8.3 respectively). Affinity estimates for tamsulosin and prazosin in antagonizing alpha1-AR-mediated contractions of human renal artery (HRA) and rat aorta (RA) were similar to those observed in LUT tissues, whereas Ro 70-0004 and RS-100329 were approximately 100 fold less potent (pA2 values of 6.8/6.8 and 7.3/7.9 in HRA/RA respectively). The alpha1A-AR subtype selectivity of Ro 70-0004 and RS-100329, demonstrated in both cloned and native systems, should allow for an evaluation of the clinical utility of a 'uroselective' agent for the treatment of symptoms associated with benign prostatic hyperplasia.