This study of Lake Tanganyika originated in the desire to apply the geochemical methods and techniques recently developed for oceanographic research to the problems encountered in trying to understand the circulation and chemistry of a large and deep lake. In many ways Lake Tanganyika resembles the oceans, especially in the 1ong "residence time" of deep water relative to replacement by mixing with surface water. This time is estimated to be of the order of 2000 years in both Lake Tanganyika and in the oceans.