Male Sprague Dawley rata were exposed to carbon fibers 7 μm in diameter and 20 to 60 μm in length, for six hours a day and five days a week for up to 16 weeks at an average chamber concentration of 20 mg/m3. Rats were killed at 4, 8, 12, and 16 weeks of exposure and after a 32-week postexposure recovery period. A similar number of control rats exposed only to air were killed at the same times. Pulmonary function tests, conducted just prior to the animals' death, did not demonstrate any significant or consistent changes. The only pulmonary finding that could be causally related to the subchronic inhalation of carbon fibers was phagocytosis of the inhaled particles by alveolar macrophages. This physiologic response was not accompanied by any local reactive pulmonary inflammation or fibrosis.