Rats were exposed to a new graphite whisker for 4 wk by inhalation. Mass and fiber concentrations were 8.3 +/- 2.2 mg/m3 and 151.7 +/- 78.8 fibers (F)/ml, respectively, and mass median aerodynamic diameter was 3.0 micro m. Geometric mean sizes of the original whiskers were 0.86 micro m diameter and 6.8 micro m length. Size distributions, numbers, and mass of deposited whiskers in rat lungs were measured at 4 days, 7 mo, and 12 mo after the end of the exposure. Lungs including the whiskers were digested with acids by microwaves, then filtered onto polycarbonate filters. The number of whiskers on the filters was counted and sizes were measured based on scanning electron microphotos, and the mass was analyzed by an x-ray diffractometer. Change in the number of whiskers in the lung was expressed as an exponential decay with time, and the longer whiskers (>20 micro m) tended to remain selectively in the lung after the 12-mo clearance. Mass of graphite whisker also decreased exponentially. Count median diameters and lengths (CMDs and CMLs) decreased gradually during clearance time, although CMDs did not decrease linearly. Morphological change of this whisker was very unique and characteristic; that is, the surface of the whiskers peeled away in the lung as a result of their stratified crystalline structure.