Abstract: Specific method of irreversible antagonism of histamine receptors, especially H.sub.1 receptors, in, for example, isolated guinea pig vas deferens by 4(5)-[2-(4-azido-2-nitroanilino)ethyl]imidazole (AAH), which is a photoaffinity analog of histamine actuated unusually by light in the visible spectrum. Additionally, compounds of the present invention are represented by the formula ##STR1## wherein R.sub.1 and R.sub.2 represent an electron withdrawing group of the formula, H, --NO.sub.2, --Cl, --F, --Br, --I, --SO.sub.3 H. When two groups occur, it is not necessary that those groups be identical. In preferred compounds R.sub.1 is --NO.sub.2 and R.sub.2 is H and R.sub.1 is ortho to amine group.R.sub.3, R.sub.4, R.sub.5 and R.sub.6 represent a proton or lower alkyl of straight and branched carbon chains. In preferred compounds R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are protons or methyl groups with not more than two of the four positions substituted with an alkyl group. Preference is given for R.sub.4 and R.sub.5.