Two amphetamine metabolites, p-hydroxyamphetamine (p-OHA) and p-hydroxynorephedrine (p-OHN), selectively inhibited the A form of monoamine oxidase (MAO) in rat and mouse forebrain homogenates. Of these two metabolites, p-OHA inhibited MAO-A more strongly than p-OHN. This MAO-A-selective inhibition by p-OHA or p-OHN was found to be competitive with respect to deamination of its substrate, 5-hydroxytryptamine (5-HT). The degree of MAO-A inhibition was not changed by 90 min of preincubation of the enzyme preparations with either metabolite, and the activity inhibited by p-OHA after the preincubation recovered completely to the control level after repeated washing. Uptake of 5-HT or dopamine into mouse forebrain synaptosomes was highly reduced by both p-OHA and p-OHN. Both metabolites were more potent in reducing dopamine uptake than in reducing 5-HT uptake. In reduction of 5-HT and of dopamine uptake, p-OHA was more potent than p-OHN. These results indicate that p-OHA is a more selective inhibitor of brain MAO-A activity and 5-HT uptake than its subsequent metabolite, p-OHN. These two actions of p-OHA might, together with possible 5-HT efflux into the synaptic cleft, greatly contribute to head twitch, a brain 5-HT-mediated animal behavior induced by p-OHA.