The Testis: Advances in Physiology, Biochemistry, and Function, Volume IV, provides an overview of the state of knowledge in the physiology, biochemistry, and function of the testis. This volume updates those areas of greatest research activity and introduces in a more complete manner those topics which have developed as subject areas in themselves. It includes a chapter on testicular steroidogenesis, which updates and expands the chapter appearing in Volume II. In addition, chapters on the role of FSH in the testis, the specialized (largely endocrine) functions of the Sertoli cells, and the entire account of the tubular hormone inhibin have all been extensions of material in the original chapter on testicular endocrinology. Similarly, separate chapters on blood flow in the testis, fluid secretion, and the blood-testis barrier all report data on subjects largely unsuspected when Volumes I-III were published. Neither the first three volumes nor is the present one intended primarily for the nonprofessional biologist or the popular reader. The coverage should be most useful and informative to professional biologists. It is anticipated that this volume will also be of interest to advanced students of animal biology as an authoritative, comprehensive, and convenient review of significant recent information concerning the testis.