The effect of 24-hr water deprivation and subsequent drinking on systemic fluid balance was determined in rhesus monkeys prepared with indwelling cardiac catheters. Significant intracellular and extracellular depletions, as indicated by increased plasma sodium concentrations, osmolality, and plasma protein concentrations, resulted from the deprivation. An early attenuation in rehydrational drinking rate (2--4 min) was not associated with changes in systemic fluid balance, which suggests presystemic influences on behavior at this time. When drinking terminated (10 min), however, plasma dilution was significant. In experiments in which monkeys were sham drinking (open gastric cannula), water but not isotonic saline infusions, given through an intestinal cannula, reduced drinking rate and produced significant plasma dilution. Intravenous water infusions reduced drinking to only a comparable extent despite more rapid and substantial plasma dilution. Thus, systemic absorption does not account entirely for the effect of intestinal water infusions on drinking. It is concluded that stimulation of mechanisms both presystemically (within the intestine or the hepatic portal circulation) and systemically is important in the control and termination of rehydrational drinking in this species.