1 To elucidate the mechanism by which amphetamine produces a functional inhibition of dopamine beta-hydroxylase (DBH), the actions of amphetamine on amine uptake and beta-hydroxylation were examined in isolated adrenal storage vesicles and in whole brain synaptosomes of the rat. 2 Amphetamine produces a competitive inhibition of catecholamine incorporation into adrenal vesicles with a Ki of 279 muM; this action of the drug accounted for slightly less than half of the total inhibition of beta-hydroxylation in the vesicles, indicating that the interference by amphetamine of access of substrate to the enzyme compartment plays an important role in functional inhibition of DBH. The remainder of the effect in vesicles probably represents direct inhibition of the enzyme. 3 In synaptosomes, similar actions on uptake of substrate and on functional enzyme activity were noted, indicating that amphetamine-induced inhibition of the neuronal amine pump can also affect DBH activity if the substrate is supplied exogenously. 4 In addition to the effects of amphetamine in vitro, chronic administration of amphetamine to rats produced an increase in total activity of adrenal DBH. 5 The net effect of amphetamine on DBH activity thus represents the summation of direct inhibition of the enzyme, indirect inhibition via reduced access of substrate and enhancement of activity via trans-synaptic induction.