preparations of dopamine beta-monooxygenase containing a full complement of copper (4.2 copper atoms per tetramer) show increased ascorbate-supported catalytic activities after addition of an excess of copper ions. The significance of this observation on the question of the number of copper atoms per active site is discussed. Low molecular weight copper complexes such as copper salicylate cause uncoupling of electron transport from hydroxylation. This uncoupling is probably the reason for the well-known inhibition of this enzyme observed at high copper concentration. The onset of inhibition by the copper chelator bathocuproine disulfonate occurs on a faster time scale than the removal of enzyme-bound copper. Nevertheless, the copper removal is sufficiently rapid to require that it be considered in interpretation of inhibition experiments with chelators.