The aim of this work was to determine the effect of various cadmium and copper concentrations on

the activated sludge dehydrogenase activity. The investigations were carried out in six aerated chambers with

activated sludge, volume of 1L each, by the continuous culture method (one control chamber, not contaminated

with heavy metals and five with 0.5; 1; 2; 4; 8 mg L-1 Cu+2 and 0.1; 0.3; 0.9; 2.7; 8.1 mg L-1 Cd2+). Cadmium

sulfate and copper sulfate as a source of heavy metals were used. The concentrations of these metal ions, causing

50% dehydrogenase activity inhibition were determined. The particular attention was paid to the toxic effect of

metal ions, as well as the variations of the microbial respiration activity proceeded during toxins exposition. The

investigation showed that even the lowest concentration of the investigated metal ions caused significant changes of the activated sludge dehydrogenases activity. Copper ions showed to be more toxic than cadmium ions.