As an approach to identification of the organic cation transport system in brush-border membranes, we designed a photoaffinity probe, 1-cyano-2-(4-azido[3,5-3H]benzoylethyl)-3-[2-[[(5-methyl-4-imidazo lyl ) methyl]thio]ethyl]-guanidine ([3H]AMC) based on the molecular structure of cimetidine, which is taken up by the organic cation transport system in brush-border membrane vesicles. The effect of nonradioactive 1-cyano-2-(4-azidobenzoylethyl)-3-[2-[[(5-methyl-4- imidazolyl)methyl]thio]ethyl]guanidine (AMC) on tetraethylammonium uptake was investigated in rat renal brush-border membrane vesicles. We examined the photolysis of AMC in which the azido group was converted to an active nitrene group using UV light at a wavelength of 254 nm and established a half-life of 7 s. This half-life duration did not significantly impair brush-border membrane vesicles during the exposure to light for photo-labeling. Photoaffinity labeling of brush-border membrane vesicles from the rat renal cortex with [3H]AMC resulted in the covalent incorporation of radioactivity into membrane polypeptides; an apparent 36 kDa polypeptide was predominantly labeled. Photolabeling specificity was shown by a reduction in the labeling of the 36 kDa polypeptide in the presence of organic cations, cimetidine, tetraethylammonium and N-methylnicotinamide whereas the organic anion, fur osemide, had no effect on labeling patterns. These data demonstrate that AMC, as well as organic cations, cimetidine, tetraethylammonium and N-methylnicotinamide, interact with a common 36 kDa membrane polypeptide, which may be the transport system or one of its brush-border membrane components.