We have examined the effects of alloxan on the binding of [3H]prazosin and [125I]monoiodocyanopindolol (ICYP) to plasma-membrane-enriched microsomes isolated from dog aortas and dog mesenteric arteries respectively. Preincubation of the vascular smooth muscle membranes with alloxan reduced the number of binding sites of the alpha- and β-adrenoceptors in a concentration-dependent manner, whereas the affinity of the radioligands for the adrenoceptors was not affected by alloxan. Streptozotocin, which is also a diabetogenic agent like alloxan, had no effect on the radioligand binding to these adrenoceptors under similar experimental conditions. The inhibitory effects of alloxan on binding to β-adrenoceptors were found to be highly pH-dependent. These results indicate that alloxan exerts adverse effects on cell membrane adrenoceptors in addition to those on the ion-transport function of vascular smooth muscle cell [Kwan (1988) Biochem. J. 254, 293-296], and also suggest that the primary site of action of alloxan is the plasma membrane.