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The peripheral, physiological mechanisms for the maintenance and termination of drinking have been difficult to investigate because ingested water activates both mechanisms during a bout of drinking. Thus, when drinking occurs, the time course and degree of activation of the mechanism for maintaining drinking, and the onset, time course, and degree of activation of the mechanism for terminating drinking have not been measured separately. This is a major obstacle to the development of an adequate theory of drinking behavior. The extent of this problem is made clear by asking, “Does the administration of a dipsogen increase water intake by stimulating the maintenance mechanism, inhibiting the termination mechanism or both?” The answer, of course, is that no one knows. Despite this ignorance, the work of the last decade has provided techniques, observations, and ideas that I believe can be used to answer these fundamental questions. I shall review the available evidence for these peripheral mechanisms in the rat. See the paper by Gibbs et al (this volume) and the books by Denton1 and by Rolls and Rolls2 for relevant evidence for other species.