During the years that have intervened since the third volume of the International Life Seiences Institute (ILSI) Monographs on Pathology of Laboratory Animals, Digestive System, was published, new information of interest to pathologists has developed at a rather remarkable pace. Standardized nomenclature has been proposed and gained significant acceptance during the period since the first edition and isbeing utilized on aninternational basis. Thishas resulted in improvement in communi- cationof pathologic data to reguiatory agencies and inscientific publica- tions worldwide. This monograph series and others sponsored by ILSI have had significant effects on improved communications and the inter- national acceptance of standardized nomenclature. In this second edition, new formats have been used in some instances where more appropriate for the subjects to be eovered. The format introduced in the first edition remains useful as it emphasizes the neces- sityto reeognize the morphologie features of pathologic lesions to iden- tify them precisely. This identifieation is an essential first step toward development of new insightsintopathogenetic meehanismsand their use in decisions eventually applieable to public health. Newinformation is included in thisedition on the nature and variability of preneoplastic lesions in the Iiver of laboratory rodents. Data on the aeeompanying changes in enzyme activity in affeeted Iiver cells are ex- panded, and additional information on spongiosis hepatis in the rat and its relation to spongiotic pericytoma is a further feature. In a few in- stanees, research on a pathologie entity has been Iimitedbut its recogni- tionremains important. Thisjustifiesinclusion of suchentities in the new edition.