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The behavioral effects of injections of enkephalin into the substantia nigra or dorsal raphe nucleus were studied in rats with lesioned serotoninergic structures. Lesions were produced by intracerebral administration of the neurotoxin 5,7-dihydroxytryptamine. Treated rats showed normalization of conditioned drinking reflex extinction. It is proposed that there is a tight connection between the normalizing effects of enkephalin on rat behavior in conditions of deficient brain serotoninergic system function and increases in the efficiency of presynaptic inhibition of dopaminergic neurons.