The International Life Sciences Institute (ILSI) was creat ed to promote cooperative efforts toward solving critical health and safety questions involving foods, drugs, cosmet ics, chemicals, and other aspects of the environment. The officers and trustees believe that questions regarding health and safety are best resolved when government and industry rely on scientific investigations, analyses, and reviews by independent experts. Further, the scientific aspects of an issue should be examined and discussed on an international basis, separate from the political concerns of individual companies. The Institute is pleased to sponsor this set of monographs on pathology of laboratory animals. This project will help to improve the scientific application of the techniques of pathology to the evaluation of substances in our environ ment. The worldwide distribution of the authors, editors, and editorial board encourages the expectation that inter national communication and cooperation will also be strengthened. Alex Malaspina President International Life Sciences Institute Preface This book on the endocrine system is Volume I of a set of reference books on pathology of laboratory animals, prepared by the Interna tional Life Sciences Institute in response to recommendations from its Committee on Pathology and Toxicology. One aim is to provide infor mation which will be useful to pathologists, especially those involved in studies on the safety of foods, drugs, chemicals, and other sub stances in the environment.