The acute inhalation toxicity of formaldehyde was tested in male Wistar rats (3/group) exposed to 0, 10, 20, or 30 ppm for 6 hours. No effects were noted in the 10 ppm animals, but in the 2- and 30 ppm groups, sniffing and face-washing movements were seen, along with yellowing of the abdominal hair ( around the penis), decreased plasma alkaline phosphatase activity and increased lung alkaline phosphatase activity, decreased white blood cell counts (high-dose group), congestion of the nasal and tracheal mucous membranes, and hemorrhagic foci in the lungs. Exposure of male Fisher rats (32/group) to 0, 0.3, 2.0, or 15.0 ppm for 6 hours/day 5 days/week, for 28 months led to decreased weight gain and good intake, runny noses, lacrimation, crouching, yellowing of the abdominal hair, bleeding from forelimbs, and death of 20/32 rats in the high-dose group. Squamous cell carcinoma (14/32) and papilloma (5/32) were seen in high-dose rats, but no neoplastic changes were noted in the 0, 0.3 or 2.0 ppm groups. Dose-related excessive secretion, rhinitis, desquamation, squamous epithelial metaplasia and epithelial cell hyperplasia were observed, beginning in the low-dose group.