The renal handling and effects of an intravenous bolus of frusemide with and without plasma volume expansion with dextran or albumin, and with large variations in plasma albumin concentration, have been studied in five patients with the nephrotic syndrome. Decreased renal sensitivity to frusemide was found in only one patient, who also had hypovolaemia and an activated renin-angiotensin-aldosterone system. Plasma volume expansion increased the diuresis but not the saluresis, and slightly increased renal sensitivity to frusemide. An increase in albuminuria after albumin infusion did not reduce the sensitivity to frusemide. A decrease in plasma albumin concentration from 33 g.l-1 after albumin infusion to 23 g.l-1 after infusion of dextran caused a substantial increase in the renal clearance (from 84 to 123 ml.min-1), non-renal clearance (from 72 to 138 ml.min-1), and apparent volume of distribution (from 13 to 23 l) of frusemide, probably as a consequence of an increase in the unbound fraction. The rate of urinary excretion of frusemide was highest after albumin infusion, despite the fact that its renal clearance was lowest then.