The suitability of the open, stirred-jar technique as a method to estimate oxygen-uptake due to degradation of organic waste and nitrification by suspended microbes was studied. With this procedure, both the concentration of waste and the reaeration rate can be set to values similar to those expected in the stream. Thus, the conditions in the jars resemble the deoxygenation process in the stream more closely than the BOD-bottle test. There is no need of assuming any particular type of kinetic behavior, because the procedure gives directly an oxygen-uptake curve. Long lags before nitrification were observed. In most experiments, the oxygen-uptake rate during the nitrification stage increased slowly, resembling a first-order increasing or autocatalytic behavior. When a waste with a low COD:TKN ratio was used, the oxygen-uptake rate was constant and low, resembling zero-order kinetics. The results showed that accurate estimates of oxygen-uptake can be obtained with the open stirred-jar test. The oxygen-uptake patterns include variations in the oxygen-uptake rate that are smoothed out when the classical BOD-bottle test and the first-order model are used. / M.S.