This report describes the technique and procedure for perfusing an isolated rabbit kidney with 25 ml heparinized autologous blood in a closed circuit including a pump and an oxygenator. The duration of the operative ischaemia was 5-8 min; the perfusion lasted 2.5 hours. An additional infusion was made to compensate for urinary losses. Renal blood flow increased progressively from 2.01+/-0.1 to 2.65+/-0.22* ml/g kidney weight (kw) per min (*P<0.05). Between the first (P1) and the last (P4) urine collection period the glomerular filtration rate (GFR) fell from 288+/-25 to 217+/-38* microl/g kw per min, urine flow from 5.58+/-1.13 to 4.91+/-0.75 microl/g kw per min, Na+ excretion from 1.07+/-0.19 to 0.63+/-0.12* micromol/g kw per min, K+ excretion from 0.46+/-0.03 to 0.28+/-0.05* micromol/g kw per min, P excretion from 2.5+/-0.2 to 2.0+/-0.5 microg/g kw per min, Ca excretion from 0.4+/-0.1 to 0.12+/-0.05* microg/g kw per min, creatinine excretion from 6.94+/-0.32 to 5.68+/-0.54 microg/g kw per min, glucose excretion from 18.2+/-3.2 to 1.6+/-0.5* microg/g kw per min, the free water clearance (CH2O) from -6.57+/-0.85 to -5.10+/-1.31 microl/g kw per min and urine osmolality from 600+/-52 to 590+/-105 mOsm/kg, urea excretion from 0.75+/-0.16 to 0.95+/-0.13 micromol/g kw per min. Excretion of glucose, P or Ca was observed only above a given plasma threshold value, and no transport maximum was found for glucose or P. Ca reabsorption paralleled the Na reabsorption. The proximal tubule pressure, measured within the 1st h of perfusion, was 12.5+/-1.1 mm Hg. Histological examination at the end of the perfusion showed dilatation of the tubules as in the non-perfused kidneys, and the presence of numerous bacteria. Hypertonic urine (380-1110 mOsm/kg) was observed in the presence of vasopressin, in the latter's absence the urine was hypotonic urine (206-278 mOsm/kg). There was no correlation between renal plasma flow and the GFR. CH2O increased with increasing filtered Na+ load. In conclusion, the blood-perfused, isolated rabbit kidney has a fairly constant functional capacity for approximately 2 h.