Interaction with chlorethylclonidine has been used to subdivide populations of α1-adrenoceptors in some tissues. WB 4101 can distinguish high and low affinity states of the receptor. The present study was carried out to determine if different populations or affinity states of α1-adrenoceptors distinguished by either of these compounds, could explain the variation in α1-adrenoceptor agonist affinity found amongst rabbit arteries. Five arteries were studied whose affinity for noradrenaline vary between 4.8 and 6.4. These were the thoracic aorta, renal, superior mesenteric, ear and ovarian arteries. WB 4101 was found to be equally effective in antagonizing noradrenaline on all arteries. Chlorethylclonidine caused a 20-fold rightward shift of the noradrenaline dose-contraction curve in the thoracic aorta; but had little or no effect on the other vessels. Thus, the combination of different proportionsof subsets of α1-adrenoceptors distinguished by WB 4101 or chlorethylclonidine does not explain the variation in α1-adrenoceptor affinity found in these rabbit arteries.