1. The concentration of oxalate in plasma was determined by an isotopic method involving the simultaneous measurement of [14C]oxalate activities in plasma and urine and the concentration of stable oxalate in the urine.

2. Plasma oxalate concentrations ranged from 1.3 to 1.6 μmol/l (11.8–14.3 μg/100 ml) in three normal men; in fifteen male patients with renal calcium oxalate stones the mean value was 1.73 μmol/l (15.6 μg/100 ml), SD = 0.55 μmol/l (4.98 μg/100 ml).

3. The renal clearance of [14C]oxalate ranged from 162 to 358 ml/min (mean = 249 ml/min) in the normal subjects and from 95 to 315 ml/min (mean = 201 ml/min) in the patients. A direct and statistically significant relationship was observed between the oxalate and creatinine clearances.

4. The oxalate/creatinine clearance ratio ranged from 1.42 to 2.60 (mean = 1.95) in the normal subjects and from 1.04 to 2.33 (mean = 1.76) in the patients, implying a net renal tubular secretion of oxalate. However, oxalate excretion was unaffected by probenecid, a drug known to inhibit the active tubular transport of organic anions.

5. Possible errors in the determination of plasma oxalate concentration and oxalate clearance by chemical and isotopic methods are discussed.

6. Intravenous administration of [14C]oxalate to eight subjects allowed estimations of the miscible oxalate pool [mean = 53.3 μmol (4.80 mg); SD = 18.7 μmol (1.68 mg)], the volume of distribution of [14C]oxalate (mean = 45.2% of body weight; SD = 5.65) and the biological half-life of [14C]oxalate (mean = 91.10 min; SD = 13.89).

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