Normal (N) rats and rats with nephrotoxic serum nephritis (NSN) were used in whole-kidney and micropuncture studies while being kept euvolemic by homologous plasma infusion and after isotonic volume expansion (VE). During euvolemia, whole kidney and single nephron (SN) glomerular filtration rate (GFR), as well as urinary sodium excretion (UNa X V), were significantly lower in NSN rats (GFR = 0.40 +/- 0.06 ml/min; SNGFR = 14.4 +/- 1.9 nl/min; UNa X V = 0.05 +/- 0.02 microEq/min) than in controls (GFR = 1.14 +/- 0.06 ml/min; SNGFR = 32.7 +/- 2.0 nl/min; UNa X V = 0.05 +/- 0.02 microEq/min); fractional proximal and "distal" (beyond the proximal convoluted tubule) sodium reabsorption rates were significantly higher than N, further depressing urinary sodium output. Saline expansion significantly elevated GFR and decreased renal vascular resistance (RVR) in both N and NSN rats. In the latter, however, GFR remained below, and RVR above, levels observed in N rats. Urinary sodium excretion increased markedly with saline expansion in N rats, reaching 26.9 +/- 1.31 microEq/min. NSN rats exhibited a blunted natriuretic response to expansion (UNa X V = 3.85 +/- 1.02 microEq/min); however, despite a still depressed GFR and increased RVR, urinary sodium excretion in NSN rats reached levels well above euvolemic control. Fractional proximal and "distal" tubular sodium reabsorption rates were depressed by VE in both N and NSN rats. However, this depression was less evident in NSN rats, particularly in the "distal" nephron, suggesting that the stimulus for sodium reabsorption in these segments was somehow enhanced in NSN rats. Fractional potassium excretion was increased to a much greater extent in NSN than in N rats. This finding, associated with the small depression of fractional sodium reabsorption in the "distal" nephron of NSN rats, suggests a participation of elevated levels of circulating aldosterone in the sodium retention observed in this model.