Rats, mice, guinea pigs, rabbits and dogs were exposed to butyraldehyde vapor 6 hours/day, 5 days/week for 9 days over a two-week period. The measured concentrations for the 3 test levels were 6400, 3100 and 2000 ppm. Definite signs of eye and respiratory irritation, and statistically significantly lower body weight findings were observed in most species inhaling 6400 and 3100 ppm of butyraldehyde. Other signs observed in most animals at 6400 ppm included coordination loss, anesthesia and death. At 3100 ppm these effects were observed only in the beagle dog. Only some eye and respiratory irritation and statis- tically significantly lower body weight effects were observed among animals inhaling 2000 ppm of butyraldehyde. Scattered organ weight effects were found in rats for both test groups (3100 and 2000 ppm) surviving the 9-day inhalation treatment. (Further interpretation of these organ weight findings will probably be forthcoming in the following 13 week study.) No pathologically significant treatment related gross lesions were found among animals inhaling 3100 or 2000 ppm of butyraldehyde. One male Sprague-Dawley rat that had been exposed to 6400 ppm had bilateral bemorrhage of the ethmoturbinates.