Male and female Sprague-Dawley rats of different ages at the start of the experiments (12 day embryos, and 7 and 25 weeks old) were administered fromaldehyde in drinking water at different doses (2,500 or 1,500, 1,000, 500, 100, 50, 10, 0 ppm). An increased incidence of leukemias and of gastro-intestinal tumors was observed in formaldehyde treated rats. Gastro-intestinal tumors are exceptionally rare in the rats of the colony used. These results, together with the ones obtained by other Authors on rats exposed by inhalation to formaldehyde, indicate that this compound is an experimental multipotential carcinogen. The experimental results presented in this report give scientific support to the epidemiological observation of a higher incidence of leukemias and of gastro-intestinal cancers among the people occupationally exposed.