1. We determined the alpha 1-adrenoceptor subtypes involved in adrenergic contractions of eight different blood vessels isolated from the dog. 2. Noradrenaline produced concentration-dependent contractions in all the blood vessels tested, which were competitively inhibited by prazosin, WB4101, HV723 and 5-methylurapidil. However, there was considerable difference between the vessels with regard to the pKB values for all the antagonists. The alpha 1-adrenoceptors of dog vertebral and carotid arteries had high affinity for prazosin (pKB > 9.0) but low affinity for WB4101 (< 8.5), 5-methylurapidil (< 7.5) and HV723 (< or = 8.5). By contrast, HV723 had higher affinity (> 9.0) than prazosin (< 8.3), WB4101 (< 8.7) and 5-methylurapidil (< 8.2) in the portal vein, mesenteric artery and vein, and renal artery. In the femoral artery and vein, however, the four antagonists showed pKB values in the range 8.0-8.7. 3. Chloroethylclonidine (10 microM) produced a remarkable reduction of the contractile responses to noradrenaline in the vertebral and carotid arteries as compared with those in the other vessels. Nifedipine inhibited the responses to noradrenaline in all the tissues tested, and had marked effects in the portal vein. 4. Sympathetic adrenergic contractions induced by transmural electrical stimulation were also inhibited by prazosin and HV723 at different potencies among tissues. The relative potencies of both the antagonists paralleled the relationship in inhibiting the responses to exogenous noradrenaline in each vessel. 5. According to recent alpha l-adrenoceptor subclassification, the present results suggest that the contractions of blood vessels induced by endogenous and exogenous noradrenaline are mediated through different alpha l-adrenoceptor subtypes heterogeneously distributed in each vessel; presumably, the alpha 1 B subtype in the carotid and vertebral arteries, the alpha IN subtype in the visceral region and the alpha IL subtype in the femoral region. Regionally different expression of alpha 1-adrenoceptor subtypes may be in part associated with the regional heterogeneity of sympathetic responses in the blood vessels.