Abstract This research explored the effects of haloperidol (HP) metabolites on biogenic amine uptake and release, and compared them to those of MPTP and its toxic metabolite, MPP +. In synaptosome preparations from mouse striatum and cortex, the HP metabolites haloperidol pyridinium (HPP +), reduced haloperidol pyridinium (RHPP +), and haloperidol tetrahydropyridine (HPTP) inhibited the presynaptic uptake of dopamine and serotonin, with greater affinity for the serotonin transporter. HPP + was the most potent inhibitor of dopamine uptake, and HPTP of serotonin uptake, both with IC 50 values in the low micromolar range. RHPP + was less active than the other metabolites, but was more active than the parent compound, HP. Inhibition of uptake was reversed when free drug was removed by centrifugation and then resuspension of the synaptosomes in fresh buffer, suggesting that inhibition of uptake was due to interaction with the transporters and was not due to irreversible cytotoxicity. HPP + showed noncompetitive inhibition of both serotonin and dopamine uptake, suggesting that it has a relatively slow dissociation rate for its interaction with the transporter proteins. In experiments on amine release, HPP + and HPTP were four-fold less potent than MPP + for releasing preloaded dopamine from striatal synaptosomes, and only MPP +-dependent release was antagonized by the uptake blocker, mazindol. In contrast, RHPP + displayed little ability to release either amine neurotransmitter. HPTP was about two-fold more potent than MPP + for releasing serotonin from cortical synaptosomes, whereas HPP + was less active than MPP +. The specific serotonin transport blocker fluoxetine was only able to antagonize release induced by MPP +. These results suggest that HP metabolites bind to the transporters for dopamine and serotonin, but are not transporter substrates. In contrast to their potent effects on amine release, HPP + and HPTP were unable to release preloaded GABA from cortical synaptosomes. The implications of these results concerning a possible role of HP metabolites in the development of tardive dyskinesia are discussed.