Newly developed alpha1-adrenoceptor antagonists including naftopidil are free from the "prazosin-like" side effect of orthostatic hypotension and associated symptoms. We investigated the mechanism for the differential effects of naftopidil and prazosin on the development of postural hypotension, with special attention on their selectivity for the alpha1-adrenoceptor subtype. We observed that head-up tilt caused a similar extent of drop in mean arterial pressure in control, naftopidil (1 mg/kg)- or prazosin (10 microg/kg)-treated rats; however, the tilt-induced postural hypotension was recovered within 2 min in the naftopidil-treated group, but not in the prazosin-treated group. Comparing an inhibitory effect on noradrenaline-induced contraction in the rat aorta and portal vein, we found that naftopidil was sixfold less potent in the portal vein, while prazosin showed similar potency in both tissues. Reverse transcription-polymerase chain reaction analysis showed that the expression of alpha1d-adrenoceptor mRNA predominated in the aorta, while that of alpha1b-adrenoceptor mRNA predominated in the portal vein. Using cloned rat alpha1-adrenoceptor subtypes, we found that naftopidil was selective for the alpha1d-subtype with approximately ninefold higher affinity than at the other subtypes. These results show that the pharmacological character of naftopidil, combined with the differential expression of the alpha1-adrenoceptor subtype in the artery and the vein, may partly explain the differential effect of naftopidil and prazosin on head-up tilt-induced hemodynamic responses.