Chronic exposure and postexposure experiments have been recently performed in rats to evaluate the biological responses of inhaled refractory ceramic fibers (RCF) at different concentration levels. The lung burden data in the accessory lobe of the rat lung were collected during and after different exposure and postexposure periods. The size distribution of retained fibers in the lung at different time points was also measured. We used these data to develop a mathematical model of fiber clearance from the rat lung. It was found that the clearance rate did not depend significantly upon fiber size but there was a clear dependence on lung burden. As lung burden increased, the clearance rate was found to decrease. An empirical equation was derived for the clearance rate as a function of lung burden. At low burdens, rats had a retention half-time of about 126 days for RCF compared to a typical half-time of about 60 days for insoluble nonfibrous particles.