Previous studies in this laboratory have demonstrated fiber clearance and dimensional changes in chrysotile asbestos using a rat inhalational model of short-term exposure. The purpose of the present study was to determine whether or not similar changes occurred in crocidolite asbestos fibers isolated from the lungs of rats at various intervals after termination of exposure. Fibers were recovered on a membrane filter using a sodium hypochlorite digestion-concentration technique, and the numbers and dimensions of the fibers assessed using scanning electron microscopy. The mass of crocidolite asbestos retained in the lung was then calculated. Of the respirable fraction, 19% was deposited in the lungs, and 25% of this amount was still present 1 month after exposure. These values are similar to the 23% deposition and 19% retention rates for chrysotile determined in our previous study. There was a progressive increase in mean fiber length with time postexposure (P less than 0.05), but no significant changes in the diameter of the population of crocidolite fibers retained in the lung. Thus it appears that the tendency for longer fibers to be retained within lung tissue is a characteristic shared by serpentine and amphibole asbestos fibers, whereas longitudinal splitting with progressive decrease in mean fiber diameter in vivo occurs primarily with the serpentine fibers.