Improvement of a method based on OECD 209 ''Respiration Inhibition Test'' was studied. The method involves measuring the oxygen uptake rates of activated sludge microorganisms exposed to different concentrations of inhibitors. Six organic and three inorganic test compounds listed by EPA as priority pollutants were selected and their IC50 values were determined. More reliable results could be obtained, in terms of inhibition types, by prolonging the exposure time. Instant effect measurements of inhibitors gave misleading data for determining and evaluating the inhibitory behaviours.