Calcium oxalate retention was studied in non-stone-forming volunteers. All subjects were placed on a constant diet for 5 days. After the oral administration of 10 µCi of [14C]-oxalic acid, the pattern of urinary oxalate excretion was followed for 48 h. Each subject was then given 10 µCi of [14C]-oxalic acid mixed with sufficient sodium oxalate (7.5 mg/kg body weight) to induce calcium oxalate crystalluria. Urinary oxalate excretion was then recorded for 48 h. After the administration of labeled oxalic acid (without additional sodium oxalate), 76.6 ± 5.9% of the total recovered dose was excreted by 4 h. When the labeled oxalic acid was mixed with a sodium oxalate load, 62.4 ± 8.8 % was excreted by 4 h (p < 0.01). Induction of calcium oxalate crystalluria results in the retention of oxalate in the kidney. The degree of retention varies among individuals. Differences in particle retention may help explain the differences between stone formers and non-stone formers