Experimental exposures to acetone vapor were carried out in man and dog to compare the behavior of acetone in each species. Although acetone has been studied quite extensively, low-level vapor exposures are not well documented in the literature. Exposures were conducted for 2 or 4 hours at concentrations of 100 and 500 ppm with human subjects and at 100, 500, and 1000 ppm with dogs. In both species the concentrations of acetone in the breath and blood were found to be directly proportional to the magnitude of the exposure. In the human studies casual urine samples were collected for 24 hours; no direct proportionality between urinary acetone and exposure level was found. With comparable exposures, man absorbed a greater quantity of acetone than the dog. In both species the half-life of acetone in the blood was approximately the same (3 hours). In man exercise substantially increased the extent of acetone absorption. When the exposure was extended from 2 to 4 hours, less than a twofold increase in the postexposure breath concentration of acetone was found. These studies demonstrate that, when clog experiments are performed in conjunction with human studies, fundamental data concerning the behavior of acetone can be obtained. Thus dog experiments can be performed that are representative of human exposures.