In this study, we have used an alpha 1-adrenergic receptor photoaffinity ligand, 2-[4-(4-azido-3-iodo-benzoyl)-piperazin-1-yl]-4-amino-6, 7-dimethoxyquinazoline (125I-APD), to label covalently the alpha 1-adrenergic receptor in a smooth muscle cell line. Our results indicate that in the absence of light, (125I)APD binds reversibly to a site in the DDT1 MF-2 cell membranes having pharmacological characteristics of an alpha 1-adrenergic receptor. Following incorporation of (125I)APD into partially purified membranes a single labeled band of protein with a Mr of 81 000 was visualized by autoradiography following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Incorporation of (125I)-APD into this band was affected by adrenergic agonists and antagonists in a manner consistent with an alpha 1-adrenergic interaction. Prazosin (alpha 1-selective) blocked incorporation of the label into the Mr = 81 000 protein while yohimbine (alpha 2-selective) did not. Of the adrenergic agonists, (-)-epinephrine and (-)-norepinephrine but not (-)-isoproterenol blocked labeling of the Mr = 81 000 protein. We conclude that the ligand binding site of the DDT1 MF-2 cell alpha 1-adrenergic receptor resides in a Mr = 81 000 protein.