The selectivity and specificity for alpha 1-adrenoceptor blocking activity of the optical isomers of YM-12617 have been examined in pithed, spontaneously hypertensive rats. R(-)-YM-12617 and prazosin (1 mg kg-1 p.o.) produced 360- and 88-fold rightward shifts, respectively, of the dose-response curve of control to phenylephrine, whereas S(+)-YM-12617 (1 mg kg-1 p.o.) failed to cause a shift. Based on dose ratio, R(-)-YM-12617 was 320 times more potent as an alpha 1-adrenoceptor antagonist than S(+)-YM-12617. This potency ratio corresponded to that formed in an in-vitro study. Both R(-)- and S(+)-YM-12617 hardly affected the UK-14304, angiotensin II, vasopressin and isoprenaline dose-response curves. These results suggest that R(-)-YM-12617 exerted selective alpha 1-adrenoceptor blocking activity and its activity was specific for alpha 1-adrenoceptors.