I first became interested in prolactin a little over two years ago. I was then working in Nairobi and I knew nothing about the hormone apart from its role in lactation. Professor Mohammed Hyder of the Department of Zoology in the University of Nairobi was interested in the endocrine mechanisms which enable Tilapia fish to adapt to water with a very high electrolyte content. He invited me to a seminar given by Professor Howard Bern which was largely concerned with the role prolactin plays in fluid and electrolyte balance in sub-mammalian vertebrates. This inspired me to begin a programme of research into the roles prolactin plays in man and other animals. Very few physiologists or clinicians seem aware of the multifarious effects of prolactin in mammals. This book therefore aims to give a comprehensive account of the mammalian physiology of prolactin and to make suggestions about its possible role in diseases ranging from cancer to mental illness. The two subjects which have been previously widely covered, the roles of prolactin in lactation and in rat mammary cancer, are presented relatively briefly though with a full list of references. Other subjects are dealt with more extenSively and I hope that many research workers and clinicians may find the book helpful.