The respiratory rate of the mitochondrial respiratory chain is very sensitive to many toxins; some toxins enhance, while others slow the respiratory rate. We propose, therefore, a new simple "in vitro" method in order to assess acute toxicity in water using the response of the mitochondrial respiratory chain from rat liver. On the basis of the response we subdivided all tested toxins into two categories and for each toxin the EC50I value (which is the dose of toxin necessary to halve the maximal respiratory rate) and the EC50u value (which is the uncoupler dose necessary to stimulate the respiratory rate up to fifty percent of its maximum value) are calculated. The comparison between EC50 and LC50 values obtained by fish method allowed us to conclude that our method is a good tool for predicting the toxicity response in fish.