An experimental antidepressive drug, nomifensine, was tested in simultaneous experiments as an inhibitor of the uptake of noradrenaline (NA), dopamine (DA) and 5-hydroxytryptamine (5-HT) in rat brain synaptosomes. The drug was found to be a very potent inhibitor of NA (Ki 4.7 X 10(-9) M) and DA (Ki 2.6 X 10(-8) M) uptake, but relatively weak inhibitor of 5-HT uptake (Ki appr. 4 X 10(-6) M). According to kinetic studies on dopamine uptake, the inhibition was competitive. Time course studies indicated that the percentage inhibition did not increase with time. This finding suggests that inhibition of membrane uptake rather than inhibition of storage is the mechanism of action of this drug. 3 metabolites of nomifensine were also tested as inhibitors of NA and DA uptake. The addition of a 4-hydroxy group to the phenyl ring of nomifensine slightly decreased the potency, and addition of hydroxy and methoxy groups to the positions 3 and 4 in the phenyl ring, clearly decreased the potency. The structure of nomifensine is compared to that of chlorimipramine. It is suggested that the differences in selectivity as to dopamine and 5-HT uptake mechanisms might be due to 2 conformational differences: one of the phenyl rings is freely rotating in nomifensine but not in chlorimipramine, and the tertiary amine group is in a flexible side chain in chlorimipramine but rigidly tied in nomifensine.