In 5 patients with polycystic kidney disease and creatinine clearances ranging from 4 to 40 ml/min, relationships between changes in blood pressure, sodium balance, body fluid compartments, plasma renin activity (PRA), urinary aldosterone excretion, and plasma aldosterone concentrations were studied during periods of low, medium, and high sodium intake. Total body water (TBW), total exchangeable body sodium (TEBS), and extracellular volume (ECV) were measured by isotope dilution techniques, plasma volume with Evan's blue dye, and PRA and aldosterone by radioimmunoassay. Low sodium intake reduced kidney function, blood pressure, and serum sodium, while PRA reached its highest levels. Subsequent increases in sodium intake improved kidney function and increased blood pressure. Plasma volume increased slightly and ECV markedly, while PRA dropped to 15 percent of the value noted after the low sodium intake. TBW and TEBS showed inconsistent changes. Aldosterone changes correlated closely with PRA. Blood pressure showed a negative correlation with PRA, but a positive one with body weight and cumulative sodium balance, and with plasma and extracellular volumes.it is suggested that whereas renin and aldosterone are involved in the maintenance of circulatory homeostasis during sodium loss, sodium retention causes an increase in blood pressure by concomitant changes in body fluids.