Nitrification is the most sensitive part in the biologic nutrient removal of wastewaters, with the autotrophic nitrifying biomass being approximately 10 times more sensitive than its aerobic heterotrophic counterpart. The growth rate is much lower and strongly dependent on temperature, substrate concentration, oxygen content, pH, and the presence of inhibiting components. Many kinds of inhibitory compounds are present in wastewater. Some organic compounds and also heavy metals fall within this category. The article assesses these inhibitory effects, and experiments were carried out with representative compounds, that is, Zn2+ and Cu2+ as heavy metals and Ethylbenzene, Chlorobenzene, Trichloroethylene, and Phenol as the organic compound inhibitors. Among the different techniques available to assess the toxicity of components upon the activated sludge process in general, and the nitrification step in particular, respirometry has frequently been used to quantify the degree of inhibition and to study the effects on the specific growth rate of the nitrifying biomass. The present experiments use the ISO 9509 test, which is less tedious and time-consuming and based upon the rate of nitrification (against rate of oxygen uptake rate in the OUR test). The inhibition degree is given as a relationship of dose vs. effects for certain concentrations of inhibitors. The results of these ISO experiments are compared with the results of an inhibition study based on the measurement of the growth rate using the OUR test. By way of additional illustration, the ISO test is also carried out to measure toxic effects of industrial wastewater and diluted industrial wastewater. The inhibition results are categorized according to the degree of inhibition as IC50 (concentration producing 50% inhibition), IC20 (20%), and NOEC (no observed effect concentration). The results reveal that, in general, OUR tests yield a too low indication of the toxicity, with higher IC50 values, than tests based upon measuring the rate of nitrification. The discrepancy is probably the result of the short time of contact between the toxic component and the activated sludge in the OUR procedure. Repeat OUR tests over several days, moreover, give largely different results. The tests using the rate of nitrification give a more direct indication of the inhibition effect on nitrifiers.