This monograph brings together our work concerning the relationships be- tween the hypothalamus, pituitary and testis. The studies span approximate- ly a decade of collaborative work. Over this period our contributions and those of many others have advanced greatly our understanding of many aspects of male reproductive biology. This monograph attempts to survey these developments and to highlight many unresolved issues. It also provides an opportunity to demonstrate the value of investigations which relate bio- chemical to structural parameters and to illustrate the importance of animal studies in elucidating biological principles with clinical applications. The reverse is also important since the pathophysiology of human disorders of- ten provides insight into hitherto unsuspected basic mechanisms. In any rapidly expanding field it is difficult to stop revising and extending a manuscript. It is equally difficult to quote every reference in the field but we trust that those of significance have not been omitted. Some of our former students have collaborated with us as co-authors in producing this monograph. It is equally important to recognise the contributions of our other former students and colleagues who have physically participated in the studies and without whose intellectual contributions a number of concepts would not have been elucidated. Their work is acknowledged in the text of the monograph and the extensive list of references.