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A photoaffinity probe for α 1 -adrenoceptors was synthesized and its properties examined on rat brain membrane preparations. The binding of 4-amino-2-[4-(4-azidocinnamoyl)piperazino]-6,7-dimethoxyquinazoline (ACP) to these receptors was of high affinity (K D = 1.05 nM) and reversible in the dark. A dose-dependent decrease in the concentration of [ 3 H]prazosin binding sites without a change in K D was observed when membranes were preincubated with ACP, photolyzed, and then extensively washed prior to assay. This reduction in receptor concentration was prevented by α 1 -adrenergic ligands. The specificity of ACP for α 1 -receptors was further demonstrated by its inability to compete with [ 3 H]dihydroalprenolol and [ 3 H]yohimbine binding in these same membranes. Also, the concentrations and affinity constants of β-adrenoceptors and α 2 -adrenoceptors were unaffected in membranes which had been photolyzed after preincubation with ACP. No reduction in concentration of α 1 -adrenoceptors was detected if ACP was photolyzed prior to incubation with receptors or if ACP was maintained in darkness throughout the experiment. The results suggest that ACP is a specific and sensitive photoprobe that may be useful for further studies on α 1 -adrenoceptor coupled systems and that may be particularly suited for use in cell culture work.