During the last two decades, much attention has been given by scientists to the metabolite, 5-hydroxytryptamine, otherwise known as serotonin. This metabolite, of wide and varied biological activity, occurs in living organisms, participating in and often affecting many physiological phenomena and inducing some pathological changes hazardous to human health. In the present book, the etiological role which this metabolite plays in carcinoid of the gastrointestinal tract, and in so-called carcinoid syndrome is being reviewed on the basis of voluminous literature accu­ mulated during recent years. Many phases of serotonin activity associated with the carcinoid syndrome are not defined as yet, nor are the therapeutic measures to combat the manifestations of this syndrome fully elaborated. Nevertheless, considerable progress has been made in this direction which we hope could lead to a successful therapy of carcinoid syndrome as well as of carcinoid tumor itself. Contents Chapter I 5-Hydroxytryptamine - Serotonin 1 The Occurrence of 5-HT in Nature. 1 Storage and Release of 5-HT . 3 Cardiovascular Reaction to 5-HT 6 Serotonin Nephropathy. 13 5-HT in the Brain Tissue 14 References 16 Chapter II The Oncostatic Activity of Serotonin 20 Hippophae Rhamnoides . 20 Serotonin Injection into the Tumors. 25 The Influence of Endogenous Serotonin on Transplanted Tumors. 27 Serotonin Concentration in Tumor Cells 29 References 30 Chapter III The Carcinoid 31 History, Distribution, Pathology, and Cardiovascular Complications. 31 Historical Background 31 Histopathology of Carcinoid 33 Cardiac Lesions 35 Distribution of Carcinoids . 36 Age and Carcinoid Incidence 38 Sex and Incidence .