Male beagle dogs and male and female Sprague-Dawley rats were exposed 6 hours per day, 5 days per week for up to 14 and 13 weeks, respectively, to butyraldehyde vapor concentrations of 5.44, 1.36, and 0.34 mg/liter (2000, 500 and 125 ppm). The measured parameters for toxic response included body and organ weights, urinalysis, food chemistry, pathology, ophthalmologic and hematologic examinations. Rats at all levels tested had a significant incidence of aquamous metaplasia of the nasal cavities. Dogs exposed to 2000 ppm had significant microscopic lesions of the upper respiratory tract, including mucosal cell hyperplasia, inflammation, and mucous metaplasia. Exposed dogs at 500 and 125 ppm had goblet cell hyperplasia within the nasal mucosa. There were no other significant differences found between test and control groups which could be related to inhalation of butyraldehyde vapor concentrations. This study was designed to examine toxic response in dogs and rats subjected to repeated inhalation of atmospheres containing butyraldehyde vapor. All raw data are being held at the Chemical Hygiene Laboratory of Carnegie-Mellon Institute of Research for future reference.