Edema formation in nephrotic syndrome has been attributed to intravascular volume depletion resulting from leakage of plasma water into the interstitial space and activating secondary renal sodium retention. However, clinical studies indicate that edematous patients with nephrotic syndrome may have normal or expanded plasma volumes. We evaluated the relationship between plasma volume and edema formation in control rats and rats with chronic renal failure (CRF) produced by 7/8 nephrectomy. In each group, plasma volume and 22Na space were measured during the control period and after induction of hypoalbuminemia from passive Heymann nephritis. Rats with CRF had expanded plasma volume during the initial period (4.23 +/- 0.46 vs. 3.32 +/- 0.68 ml/100 g body wt) that became significantly more expanded (to 5.44 +/- 1.16 ml/100 g body wt) when they became nephrotic as 22Na space also increased. Plasma volume and 22Na space did not change in the sham-operated rats when nephrosis was produced. Plasma renin activity was lower in the CRF rats during the control period than in the sham-operated rats and fell significantly during the nephrotic period when edema developed. Nonnephrotic rats had a plasma colloid osmotic pressure (COP) of 17.8 +/- 4.3 mmHg compared with 8.5 +/- 2.9 mmHg when nephrotic. Despite this large difference in COP, both nephrotic and nonnephrotic rats exhibited the same relationship between plasma volume and extravascular sodium space, a measure of edema formation. Hypoproteinemia is not sufficient for edema formation in the rat with passive Heymann nephritis; concomitant plasma volume expansion resulting from CRF is a necessary additional component.