Abstract: Waste water to be analyzed is brought into a reactor continuously supplied with samples of activated sludge taken from the biological treatment plant; oxygen is also constantly supplied to the reactor and the content of oxygen dissolved inside the reactor is measured in order to automatically regulate the waste water flow admitted into the reactor in relation the measured value of dissolved oxygen content and thus keep the load applied to the reactor substantially constant; a toxicity alarm is released in case of abnormal reduction of respiration when the waste water admission flow is maximum.