The brain uptake of amines that do not enter the brain or enter it poorly was promoted by noncompetitive inhibitors of monoamine oxidase, as shown by behavioral and chemical criteria. Mice pretreated with water or enzyme inhibitors other than those mentioned were placid after receiving dopamine (3,4-dihydroxyphenethylamine). Mice pretreated with monoamine oxidase inhibitors (nialamide or iproniazid) showed upon treatment with dopamine the brisk motor responses characteristic of treatment with its precursor, L-dopa (3,4-dihydroxyphenylalanine). After receiving dopamine, intact nialamide-pretreated mice showed marked increases of brain dopamine, in contrast to water-pretreated test mice or water-treated controls. In unilaterally caudectomized, nialamide-pretreated mice, dopamine induced marked lateral curving of the body toward the lesion followed by running in that direction. Noradrenaline or adrenaline induced curving in caudectomized mice, whereas intact ones remained placid.