Only the results of the physics-chemical analysis isolated do not show the environmental impact caused by the pollutants demonstrated through the effects on the ecosystem. Through the tests of toxcity the potential toxical of the chemical substances are put against the system of self protection of test-organisms, that react to the global effect of the substances present in the environment, indicating the effects of these, releasing information about the quality of the environment and about the existing impact. The control of the bathes in anatomic tanks as a whole can be made through the tests of toxicity, in which the aquatic organisms are exposed to various concentrations of this effluent. The application of the tests of toxicity in the environmental analysis is enough comprising and its importance increases with the accomplishment of the monitorings. About 4,5 m³ per year of effluents containing formaldehyde need the evaluation before the final discard. The exposure to the formaldehyde of the students and workers is conditioned to the level of concentration which is nowadays 10%. It has caused health problems and is under suspect of being carcinogenic. The formaldehyde also has effects in the environment, its flowing through the sewer system can cause fire, besides it is corrosive. The knowledge of the toxicity of the effluent was obtained, through the methods standarized as the test with Dapnia similis (aquatic organism) and Vibrio fischeri (bacterium) because of the quickness of the results. To avoid the toxical effect it is necessary a factor of dilution of the bathes between 1000 to 6400, for aquatic organism and bacterium respectively.