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A new compound, azapride, i.e. 4-azido-5-chloro-2-methoxy-N-[1-(phenylmethyl)-4-piperidinyl]benzamide, the azide derivative of the dopamine antagonist clebopride, was synthesized and tested for its usefulness as a photoaffinity probe for dopamine receptors. Without photolysis the azide behaved as a very selective dopamine antagonist. When a microsomal preparation of dog striatum was pretreated with the azide and subsequently irradiated with ultraviolet light, dopamine receptors became irreversibly blocked. This reaction was dependent on both azide concentration and time of irradiation. The irradiation-induced inhibition was quite selective for the dopamine receptor; various other receptor systems were not affected. Moreover the irreversible inhibition of dopamine receptors from dog striatum could be prevented by dopamine antagonists as well as by a dopamine agonist, but not by a serotonin antagonist. The irradiation-induced inactivation by azapride remained unchanged in the presence of the scavenger p-aminobenzoic acid. These findings provide evidence that azapride labels dopamine receptors specifically and irreversibly and by a true photoaffinity mechanism. It should therefore be of great value for further molecular characterization and purification of dopamine receptors. The identification of brain dopamine receptors has been closely associated with the development of neuroleptic drugs: they were for the dopamine receptor what cc-bungarotoxin was for the nicotinic receptor [l]. Indeed, in contrast to hormones and neuropeptides, classical neurotransmitters (acetylcholine, dopamine, 5-hydroxytryptamine, noradrenaline, etc) cannot be used to characterize receptor sites in vitro because of their very low affinity and rapid rate of dissociation 121. Biochemical and pharmacological studies first suggested the existence of dopamine receptors 13, 41; more direct evi