A study of the selective stimulation of respiration in the cultures of bacteria and protozoa, as found in a typical domestic waste water, was conducted in the Sanitary Engineering and Science Laboratory of The Rice Institute during the 1958-59 academic year. These studies were part of a broad investigation of the study of the progression of the biochemical oxygen demand of soluble organic compounds in order to determine the validity of a short-term B. O. D. determination. The progression in soluble substrates of the B. O. D. exerted by a mixed culture of micro-organisms was found to be a multi-stage reaction. This is believed to be the first work using this biochemical technique of studying intermediary metabolism to define the progression of the biochemical oxygen demand of soluble organic compounds. Previous work on the inhibition of assimilation utilized pure cultures and heavy inoculums of bacteria for short incubation periods. Sodium azide, 2, 4-dinitrophenol, and sodium fluoride were used for the selective stimulation of respiration or inhibition of assimilation processes. Sodium azide was the only agent which produced a marked degree of stimulation of respiration for either glucose, glutamic acid or a 1:1 mixture of these substrates. The general pattern of oxidation due to these agents is shown in graph form. The progression of the oxidation of these soluble compounds can be altered considerably and yet produce 120-hour incubation period (five-day) B. 0. D. values which are within the accepted standard deviations for these substrates using the bottle dilution technique. Similar results were obtained from manometric experiments. A critical concentration of the inhibitory agent significantly prevented synthesis and forced the reaction in the direction of complete oxidation to carbon dioxide and water, as shown by increased plateau values. Low concentrations of sodium azide initially retarded respiration slightly and simultaneously the synthesis processes were partially inhibited. Relatively high concentrations of this agent reduced the rate of respiration considerably. No increase in the extent of oxidation of fresh settled sewage was observed in the presence of sodium azide or 2, 4- dinitrophenol.