Although selected heavy metals (HMs) stimulate biological reactions at low concentrations, all HMs are toxic to microorganisms (MOs) at moderate concentrations and can cause inhibitory effects on the biological processes. Therefore, MOs must be acclimated to HMs or other toxic substances present in wastewaters (WWs) before they are used in an activated sludge process (ASP). In this study, combined effect of Cu2+ and Zn2+ ions in a synthetic WW on the efficiency of a laboratory- scale ASP without recycle was investigated using acclimated MOs at different extents. A synthetic feed solution of 1222 mg L-1 proteose-peptone (corresponding to 1300 mg COD L-1) served as a source of carbon. Cu2+ and Zn2+ ions at different concentrations (1.5, 4.5 and 9, 27 mg L-1, respectively) were introduced in the feed to a continuously stirred activated sludge reactor at different hydraulic residence times (2-40 h) keeping pH, temperature and stock feed composition constant. The combined effects of copper and zinc ions were determined by mixing these metallic ions at the specified combinations of concentrations such as "1.5 mg L-1 of Cu2+, + 9 mg L-1 of Zn2+,, and "4.5 mg L-1 of Cu2+ + 27 mg L-1 of Zn2+". It was observed that using seed MOs acclimatized to two times of the combined threshold concentration of these HMs for an unduly long period of time (1-4 months) caused adverse effects on the ASP performance. Besides, it was found that usual inhibition effects of these HMs were enhanced with increasing period of acclimation. Substantially lower substrate removal efficiencies were obtained with acclimatized MOs than those obtained with non-acclimatized MOs. At the higher initial substrate concentration of 2500 mg COD L-1, substrate-inhibition occurred causing a decrease in the specific growth rate constant (k); however, HM inhibition was suppressed, resulting to about 20% increase in treatment efficiency of the ASP. It can be concluded that the time period necessary for acclimatization of seed MOs must be adjusted carefully with concentrations of HMs lower than their threshold concentrations to achieve an optimal operation of an aerobic biological process. (c) 2006 Published by Elsevier B.V.