The influence of solid retention time, cadmium loading and the effect of the process of acclimation on the activity of activated sludge in the presence of the metal were studied. Activated sludge was cultivated in plug-flow and continuous completely mixed lab-scale bioreactors. Respirometric measurements were applied to evaluate the inhibition effects of cadmium on activated sludge. Monod's equation and the equation of non-competitive inhibition were applied to describe the toxicity and inhibition of cadmium. Maximum inhibition effect of cadmium was observed at different time periods of cadmium acting on both acclimated and non-acclimated sludges. The dependence of the attainment of maximum inhibition effect on the metal loading as well as solid retention time was observed. Therefore, maximum inhibition should be included in the assessment of metal toxicity.