Plasma and urine sodium, potassium, osmolality, aldosterone, and plasma renin activity (PRA) were measured in time controls and during control dehydration and rehydration periods in conscious dogs fed a 50 mmol NaCl diet. During 4 days of water restriction, plasma sodium and osmolality and PRA were elevated, whereas plasma potassium and aldosterone were not affected. Urinary potassium excretion was elevated above intake levels during the entire dehydration period, whereas Na excretion was only elevated during the first 2 days of dehydration. Unrestricted rehydration was associated with a marked fall in plasma sodium and osmolality, a decrease in PRA, and no change in plasma potassium or aldosterone. In addition, during the 1st day of rehydration, there was marked retention of sodium and potassium. Thus, in the dog, dehydration and subsequent rehydration cause significant alterations in sodium and potassium balance and marked alterations in PRA that are not associated with changes in either plasma aldosterone concentration or urinary aldosterone excretion.