&NA;Prolonged stimulation of tissues by adrenergic agonists may lead to diminished responsiveness of the tissues to subsequent activation by catecholamines; this phenomenon has been termed desensitization or tachyphylaxis. We have examined the in vivo consequences of prolonged stimulation of vascular &agr;‐adrenergic receptors in rats harboring pheochromocytoma, a tumor that secretes catecholamines. In both early (3‐4 weeks after implantation) and late (6‐7 weeks after implantation) stages of tumor development, New England Deaconess Hospital rats with transplanted pheochromocytomas developed hypertension and tachycardia and had plasma dopamine and norepinephrine concentrations markedly greater than controls. In both these stages of pheochromocytoma, pressor responses to several vasoconstrictors were examined after pithing. Rats with the tumor were found to become progressively subsensitive to &agr;‐adrenergic agonists. In the early phase of pheochromocytoma, loss in sensitivity was found for both &agr;1‐ and &agr;2‐adrenergic agonists, whereas responsiveness to the nonadrenergic vasoconstrictors Arg‐vasopressin and angiotensin‐II was intact (homologous desensitization). However, in the later stage of pheochromocytoma, pressor responses to all these vasoconstrictive agents and also to stimulation of the complex sympathetic outflow were found to be subsensitive (heterologous desensitization). In plasma membranes prepared from mesenteric arteries of early stage tumor‐bearing rats, [3H]prazosin binding sites were significantly decreased to 150 ± 12 fmol/mg vs. 234 ± 19 fmol/mg in controls. [3H]Yohimbine binding sites were not significantly altered. Our results show that both postjunctional &agr;1‐ and &agr;2‐adrenergic receptor‐mediated vasopressor responses can be specifically attenuated in the presence of chronically elevated endogenous catecholamine levels produced by pheochromocytoma and that each &agr;‐receptor subtype may be differentially regulated in the development of desensitization. (Circulation Research1987;61:86‐98)