1. alpha 1-Adrenoceptor subtypes in rabbit thoracic aorta have been examined in binding and functional experiments. 2. [3H]-prazosin bound to two distinct populations of alpha 1-adrenoceptors (pKD,high = 9.94, Rhigh = 79.2 fmol mg-1 protein; pKD,low = 8.59, Rlow = 215 fmol mg-1 protein). Pretreatment with chloroethylclonidine (CEC, 10 microM) almost inactivated the prazosin-high affinity sites and reduced the number of the low affinity sites without changing the pKD value. 3. In the displacement experiments with CEC-untreated membranes, unlabelled prazosin, WB4101 and HV723 displaced the binding of 200 pM [3H]-prazosin monophasically; the affinities for WB4101 (pK1 = 8.88) and HV723 (8.49) were about 10 times lower than that for prazosin (9.99). In the CEC-pretreated membranes also, the antagonists inhibited the binding of 1000 pM [3H]-prazosin monophasically; the pK1 values for prazosin, WB4101 and HV723 were 9.09, 8.97 and 8.17, respectively. These results suggest that the prazosin-high and low affinity sites can be independently appraised in the former and latter experimental conditions. Noradrenaline, but not methoxamine, showed slightly higher affinity for the prazosin-high affinity site than for the low affinity site. 4. In the functional experiments, noradrenaline (0.001-100 microM) and methoxamine (0.1-100 microM) produced concentration-dependent contractions. Pretreatment with CEC inhibited the contractions induced by low concentrations of noradrenaline but without effect on the responses to methoxamine. Prazosin inhibited the concentration-response curves for noradrenaline in the CEC-untreated aorta in a manner which was not consistent with competitive antagonism at a single site, and two distinct affinity constants(pKB = 9.71 and 8.74) were obtained. However, after CEC-pretreatment, Schild plots for prazosin were not significantly different from unity (pKB = 8.50). WB4101 and HV723 competitively inhibited the noradrenaline-induced contraction with low pKB values (approximately 8.30), irrespective of CEC pretreatment.Methoxamine-induced contractions were competitively inhibited by prazosin, WB4101 and HV723 with low pKB values similar to those obtained when noradrenaline was used as the agonist.These were not affected by CEC-pretreatment.5. The affinity constant for noradrenaline (pKA = 6.40) in CEC-untreated aorta was slightly greater than that obtained in CEC-pretreated aorta (5.78). On the other hand, methoxamine showed a similar affinity in CEC-untreated and pretreated aortae (pKA = approximately 4.5).6. Nifedipine (1 microM) slightly attenuated the contractile responses to noradrenaline and methoxamine in CEC-untreated and pretreated aortae, suggesting that nifedipine cannot discriminate between alpha 1-adrenoceptors involved in CEC-sensitive and -resistant contractions.7. From these results it is suggested that in the rabbit thoracic aorta there are two distinct alpha 1-adrenoceptor subtypes (presumably alpha 1B and alpha 1L subtypes according to recently proposed subclassification),both of which are involved in noradrenaline-induced contraction. The alpha 1L subtype predominantly mediates the contraction induced by methoxamine.