The effect of nomifensine on the uptake of 5-hydroxytryptamine (5-HT) and dopamine (DA) into human platelet-rich plasma has been studied. 2. A significant inhibition of DA uptake was observed at nomifensine 10(-6) M. 3. 5-HT uptake was significantly inhibited only at nomifensine 10(-4) M or more. 4. These results are consistent with those from previous animal experiments and suggest that the human platelet may be used as a model for studying DA uptake as well as that of 5-HT.