Rats were exposed to NIEHS short-range and to NIEHS intermediate-range chrysotile fibres for periods varying from one hour to one year. Following the exposures, the lungs from each animal were fixed by intratracheal instillation of 2% glutaraldehyde. Studies of fibre location following short exposure periods suggest that inhaled asbestos reaches the connective tissue compartment of the lung by passing through epithelial cells. X-ray energy microanalytical techniques were used to determine the magnesium to silicon ratios in fibres in situ. Inhaled particles were found to have decreased Mg:Si ratios if they were located in the interstitium, and many of the particles located within the alveolar airspaces had an even more marked decrease in the Mg:Si ratio. Morphometric techniques were used to quantitate structural changes occurring in the asbestos-treated animals. After three months of exposure, both types of chrysotile asbestos fibres caused similar increases in the volume of the alveolar epithelium, the interstitium and in alveolar macrophages. After twelve months of exposure, greater lung injury, involving both the alveolar epithelium and the interstitium, was found in animals exposed to the intermediate-range fibres. Physiological studies done after one year of asbestos exposure demonstrated a fall in total lung capacity and in vital capacity, which was most marked in the animals exposed to the intermediate-range fibres. These studies suggest that both classes of chrysotile fibres cause lung injury but that long fibres have the greater toxicity.