This paper reviews publications dealing with the deposition of fibrous particles, including asbestos and man-made mineral fibres, in the respiratory tract of man and experimental animals, particularly of the rat. The effects of fibre diameter and length on total, thoracic and alveolar deposition are discussed. Total deposition in the respiratory tract of the rat increases quite steeply with aerodynamic diameter (Dae) from about 20% at a Dae of 1 micron to 100% at a Dae of 5 microns. Deposition in the alveolar region reaches a peak of about 10% at a Dae of about 2 microns, which corresponds to an actual fibre diameter of about 0.4 microns. For fibres with diameters greater than this, alveolar deposition falls rapidly. For example, long glass fibres with an actual diameter of 1.5 microns or short glass fibres with an actual diameter of 3 microns are essentially non-respirable in the rat. The fate of fibres deposited in different regions of the respiratory tract of the rat is also discussed and the factors which predispose fibres either to remain in alveolar macrophages or to be transferred to the interstitium and pulmonary lymphatics. Finally, the distributions in the lungs of fibres administered by inhalation and by intratracheal instillation are compared, and the advantages and drawbacks of each method of delivery discussed.