The evaluation of the toxicity of mineral fibres has been tried to achieve in experimental animal models. However, the appearance of fibres in the pleural space could not be explained satisfactorily. Histomorphological examinations showed that intratracheal instillation of asbestos fibres leads to parabronchial and intraalveolar granulomatous tissue reactions and bronchial epithelial regenerations. For further elucidation of the pathogenesis of lung cancer and of mesothelioma the localisation and transport of inhaled fibres is of high interest. Thus, a three dimensional visualization of the structure of rat lungs before and after intratracheal instillation of UICC crocidolite fibres was performed by plastic casts to follow the way of asbestos fibres in the lung tissues and the pleura. The casts allowed to demonstrate airway structures with imprints of epithelial cells and blood vessels of normal and treated animals by scanning electron microscopy. Instilled asbestos fibres transformed bronchial structures and resulted in cystic deformations of the pleural surface. The penetration of single fibres through bronchial trunks and the visceral pleura could be shown for the first time in a three-dimensional topography of the affected tissue. Now, there is support for similar results of histomorphological examinations indicating the possibility that asbestos fibres could penetrate the pleura and migrate into the pleural space. The question if the migration of fibres is a mechanical movement or an active transport is still under discussion.