To determine the role of hypoxemia in the pathogenesis of impaired sodium and water excretion in advanced chronic obstructive lung disease, 11 clinically stable, hypercapneic patients requiring long-term supplemental oxygen were studied. The renal, hormonal, and cardiovascular responses to sodium and water loading were determined during five-and-a-half-hour studies on a control day (arterial oxygen tension = 80 +/- 6 mm Hg) and on an experimental day under hypoxic conditions (arterial oxygen tension = 39 +/- 2 mm Hg). Hypoxemia produced a significant decrease in urinary sodium excretion but did not affect urinary water excretion. Hypoxemia also resulted in concomitant declines in mean blood pressure, glomerular filtration rate, and filtered sodium load. Renal plasma flow and filtration fraction were unchanged whereas cardiac index rose. On the control day, plasma renin activity and norepinephrine levels were elevated whereas aldosterone and arginine vasopressin levels were normal; none of these four hormones was affected by hypoxemia. Renal tubular function did not appear to be altered by hypoxemia as there was no significant change in fractional reabsorption of sodium. The concurrent decreases in glomerular filtration rate, filtered sodium load, and mean blood pressure at constant renal plasma flow suggest that the reduction in urinary sodium excretion was due to an effect of hypoxemia on glomerular function, possibly related to impaired renovascular autoregulation.