The time course and mechanism of the natriuresis that accompanies 24 h of water, but not food, deprivation were studied in eight chronically catheterized dogs. Dogs were fed a controlled diet containing 35 meq of sodium, 110 meq of potassium, and 107 ml of water at 9:00 A.M. every day (time 0), and urine and blood samples were taken at 0, 2, 4, 6, 8, 10, 12, 16, 20, and 24 h after feeding on 3 consecutive days: the control, dehydration, and recovery days. Twenty-four hours of water deprivation decreased body weight by 880 g, increased plasma sodium by 7.2 meq, and increased plasma osmolality by 22.5 mosmol/kg. Cumulative daily sodium excretion increased from 30.4 +/- 2.8 meq on the control day to 50.7 +/- 5.7 meq on the dehydration day (P less than 0.01) and resulted in significant negative sodium balance that developed during the last half of the dehydration day. Cumulative sodium excretion during the first half of the dehydration day was not different from the corresponding period of the control day. Cumulative potassium excretion also increased from 91.9 +/- 4.5 meq during the control day to 123.0 +/- 6.7 meq during the dehydration day (P less than 0.01). Significant and progressive increases in plasma vasopressin concentration and renin activity were observed during water deprivation, but plasma aldosterone did not change from control levels. Furthermore, there was no correlation between the rate of sodium excretion and plasma vasopressin or aldosterone concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)