Fluid restriction, sodium depletion, hypotensive hemorrhage, and internal shifts in body fluids are known to be followed by increased renin and aldosterone secretion. Comparably little is known about the secretion of vasopressin (AVP), another hormone regulating body fluid. Following the recent development of sensitive radioimmunoassays, data were presented on the effect of dehydration on AVP levels in man (1, 6, 14) and in rats (3). No significant rise in AVP levels was observed during short-term dehydration in man. The present study was designed to reevaluate the influence of fluid restriction on AVP levels; the rat was used as an experimental animal, since fluid deprivation could be prolonged for several days in this species. To our knowledge, no study is presently available which evaluates all the hormones involved in the regulation of body fluids during water deprivation. In the present study, we report on AVP, aldosterone, plasma renin activity, and corticosterone in rats following water deprivation, since the recent development of sensitive radioimmunoassays (RIA) has made determinations from a limited amount of plasma possible.