The pharmacokinetics of oxalate were studied in normal and nephrectomized rats, using radioisotope-labelled oxalate to resolve the mechanism of calcium oxalate stone formation. Plasma disappearance of 14C-oxalate was analyzed with a 2-compartment open model, and each compartment volume, the first-order rate constant for elimination, and the first-order rate constant for transfer between the central compartment and peripheral compartment were obtained. These values were compared with those for inulin. In normal rats, the total distribution volume was 57% of body weight for oxalate and 34% for inulin. The elimination rate constant from the central compartment for oxalate was lower than that for inulin, but oxalate had a much larger central compartment volume than inulin. Thus, the total clearance of oxalate was greater than that of inulin. In nephrectomized rats, total clearance of oxalate decreased to 1/6 of that in normal rats, while total clearance of inulin decreased to 1/27 of that in normal rats. These results suggest that oxalate is more diffusible than inulin, and that oxalate is excreted mainly from the kidney.