With phenol and two syntans (synthetic tanning agents) as examples, a description is given of possible ways to observe inhibitory effects of water-soluble xenobiotics on the respiratory activity of microorganisms of activated sludge using two procedures: the respirometric method after ISO 8192 or manometric method of determination of biochemical oxygen demand. Description of inhibition curves and calculations of effective concentrations, EC50, using the equation of a normal distribution function was satisfactory in both methods. Attention was mainly focused on evaluation of the influence of exposure time on inhibition expressed by EC50 values. After 1 h exposure, the inhibitory effects decreased in the order Kortan FB > Kortan MX > phenol. At 15 h exposure, the order was virtually reversed, inhibitory effects of syntans on respiration were negligible, and the phenol toxicity grew with the contact time. The ISO method proved suitable for determination of inhibition after a short contact time (1 h) under "standard" conditions; the manometric test, on the contrary, for assessment of changes in inhibitory effects during a long-term contact of microorganisms with tested substances and thus for studying possible adaptations and behaviour in real conditions of biological activation.