Formaldehyde was administered for 104 weeks in drinking water supplied ad libitum at concentrations of 1500, 1000, 500, 100, 50, 10, or 0 mg/L to groups of 50 male and 50 female Sprague-Dawley rats beginning at seven weeks of age. Control animals (100 males and 100 females) received tap water only. Acetaldehyde was administered to 50 male and 50 female Sprague-Dawley rats beginning at six weeks of age at concentrations of 2,500, 1,500, 500, 250, 50, or 0 mg/L. Animals were kept under observation until spontaneous death. Formaldehyde and acetaldehyde were found to produce an increase in total malignant tumors in the treated groups and showed specific carcinogenic effects on various organs and tissues.