A brief account of the development and current state of our knowledge of the caudal neurosecretory system (urophysis) of fishes and its neurohormones, the urotensins, is presented. The structure of the system and the biological actions and chemistry of its active factors are discussed, in an attempt to assess their possible physiological significancefor the organisms which possess them. The semihistorical presentation is intended to follow the course of discovery in a readily circumscribed area of organismal biology.