1. In previous studies we found that albumin infusions caused only a modest natriuresis in the nephrotic syndrome, suggesting that hypovolaemia played no part in the sodium retention of these patients. However, this finding was inconclusive, since the hyperoncocity of the infused albumin probably opposed sodium excretion.
2. In the present study, we examined the effect of sustained (68 h) plasma volume expansion (+18%), by means of iso-oncotic albumin infusions, on renal function, blood pressure, humoral factors and sodium balance.
3. Plasma atrial natriuretic peptide levels increased almost threefold and renin-angiotensin system activity was suppressed. Glomerular filtration rate remained unchanged, whereas estimated renal plasma flow increased, resulting in a further decrease in filtration fraction.
4. The increase in plasma volume expansion was accompanied by a modest increase in sodium excretion, which, however, was less than the amount of sodium daily infused with the albumin solutions and consumed with the diet, so that net sodium was retained.
5. This observation supports the concept that an intrinsic renal defect causes the sodium retention in the nephrotic syndrome, and argues against the therapeutic use of albumin infusions.