Approximately ten years have elapsed since the second volume of the International Life Sciences Institute (ILSI) Monographs on Pathology of Laboratory Animals, Respiratory System, was first completed. New in formation of interest to pathologists has developed at a rather remark able pace during these years. Exceptional progress has been made in the routine identification of enzymes and cell products in respiratory cells. A better understanding has developed on the functions of cells of the respiratory tract and of the mechanisms involved in cell metabolism, particularly those involving toxins and carcinogens. Clear concepts have developed concerning the significance of pathologic lesions, particularly in the upper respiratory tract and their relation to human health and risk assessment. Standardized nomenclature has developed significantly dur ing the lO-year period since the first edition and is being utilized on an international basis. This has resulted in significant improvement in com munication of pathologic data to regulatory agencies and in scientific publications worldwide. This monograph series and others sponsored by ILSI have had significant effects on these improved communications and the international acceptance of standardized nomenclature. In this sec ond edition, new formats have been used where more appropriate for the subjects to be covered.