Fifty-three patients undergoing elective reconstructive surgery on the abdominal aorta were included in a prospective study to obtain information on postoperative glomerular filtration rate (GFR) in relation to increased fluid and sodium intake and decreased plasma colloid osmotic pressure (COP). GFR, extracellular fluid volume (ECV) and COP were measured before operation and on the first and fourth day after surgery. GFR and ECV were measured by means of the single injection residue detection method using the inulin analogue polyfructosan-S as marker. The compiled GFR values increased from 92.3 to 103.7 ml min-1 1.73 m-2 (P less than 0.002) on the first day, and to 105.3 ml min-1 1.73 m-2 (P less than 0.001) on the fourth day after surgery. Concomitantly, ECV increased from 8.9 litres 1.73 m-2 (P less than 0.025) respectively. Positive linear correlation was found on the first postoperative day between GFR per 1.73 m2 and ECV per 1.73 m2 (r = 0.33, P less than 0.05) and between GFR per 1.73 m2 and extracellular sodium content per 1.73 m2 (r = 0.38, P less than 0.01). COP was not correlated to GFR per 1.73 m2. The postoperative increase in GFR found in this study is most likely to be the result of increased ECV and renal plasma flow. Whether other factors are also involved remains unknown.