Elevations in the concentration of plasma angiotensin II (AII) and decline in plasma aldosterone (Ald) were noted in African Green monkeys at 48 hr of water deprivation but not subsequent to an equivalent duration of food deprivation, compared with nondeprived levels. In a second experiment, drinking was initiated following treatment with AII, hypertonic saline, and the beta-adrenergic stimulator isoproterenol. Concomitant elevations in plasma AII concentrations were measured following isoproterenol injection, but not after AII or hypertonic saline injection, when compared with isotonic saline treatment. Elevations in plasma Ald levels were noted following AII injection. A third experiment evaluated dipsogenic additivity of stimuli by comparing the volumes of water consumed following isoproterenol or hypertonic saline injection with the intake resulting from combined treatment with isoproterenol and hypertonic saline. Additivity was tested under ad lib conditions and following adaptation to a daily water deprivation regimen. The results of the first two experiments generally agree with predictions based on the respective contributions by intracellular dehydration and extracellular fluid volume depletion, to thirst. However, additivity of thirst stimuli was not demonstrated in the third experiment.