CuSO(4) and Cu(OAc)(2) were added to activated sludge to study their effect on the dehydrogenase activity for 290 min. A decrease of the activity by more than 50% was obsd. both after 5 min long exposure (2.87 mg Cu/L for Cu(OAc)(2) and 1.34 mg Cu/L for CuSO(4)) and after 290 min long exposure (1.43 mg Cu/L and 0.75 mg Cu/L, resp.). In the case when Cu concn. was lower than 0.1 mg/L, a stimulation of dehydrogenase activity was obsd. during the whole exp. time.