In a prospective randomized trial of 16 patients undergoing abdominal vascular reconstructive procedures, changes in plasma volume, serum oncotic pressure (pis), serum albumin and total protein concentration, alveolar to arterial oxygen tension differences (AaDO2, FIO2 = 1.0), creatinine clearance, body weight, and fluid and sodium intake were examined. By random assignment patients received either an albumin- or a sodium-rich intraoperative fluid regimen. Pulmonary arteriovenous admixture was significantly less in the albumin group (n = 7) than in the electrolyte group (n = 9) on the first postoperative day. The change in AaDO2 correlated positively with the total sodium intake in the electrolyte group. Despite the larger fluid load and significantly greater gain of body weight, patients in the electrolyte group had a postoperative plasma volume significantly lower than the preoperative value. Postoperative values of albumin concentration, circulating albumin mass and pis were significantly greater in the albumin group in comparison to the electrolyte group. Creatinine clearance values were not different between the two groups. The change in pis correlated significantly with sodium intake and circulating albumin mass. Pulmonary shunting and expansion of the extracellular fluid volume may be minimized without adverse effects on renal function by administration of fluids rich in albumin in preference to sodium.