An arylazide photoaffinity probe for alpha 2-adrenoceptors has been developed and characterized. The compound, 3-methyl-6-chloro-9-azido-1H-2,3,4,5-tetrahydro-3-benzazepine (SKF 102229), had a Ki for the human platelet alpha 2-adrenoceptor of approximately 40 nM. Upon exposure to ultraviolet light, SKF 102229 irreversibly blocked the binding of [3H]yohimbine to both membrane bound and solubilized, partially purified, receptors. The extent of alpha 2-adrenoceptor blockade was dependent upon both the length of exposure to ultraviolet light and the concentration of SKF 102229. Typically, a 60% decrease in alpha 2-adrenoceptor number is obtained following 8 min of photolysis in the presence of 100 nM SKF 102229. The pharmacologic characteristics of the irreversible blockade produced by SKF 102229 were those of an alpha 2-adrenoceptor. Thus, photodependent, irreversible blockade of alpha 2-adrenoceptors by SKF 102229 was prevented by the concomitant presence of phentolamine or p-aminoclonidine but not by prazosin. Given its specificity and efficient blockade of the ligand binding site, SKF 102229 should prove useful for studies of the structure and function of alpha 2-adrenoceptors.