This study was done to resolve published discrepancies in oxalate excretion between humans and rats and to characterize oxalate partitioning in rats during persistent severe hyperoxaluria, such as that seen in many bariatric patients. Osmotic minipumps dispensing 360 micromole/day KOx + 3.9 +/- 0.14 microCi/day (14)C-oxalate were implanted subcutaneously. All excreta were collected. Rats were killed on day 13 and carcasses were dissected, ground, dissolved in HCl, and subjected to scintillation counting, and 92.1 +/- 3.9% of total oxalate administered was recovered. This was partitioned among the skin complex (38.2 +/- 7.7%), carcass complex (24.5 +/- 5.9%), and excreta (29.5 +/- 1.9%). The distribution of oxalate in the skin and carcass complexes led us to infer that only 29.5 +/- 1.9% of the administered oxalate entered the circulation. Of the circulated oxalate, 98.4 +/- 0.4% was excreted (total urine 78.9 +/- 1.7%; raw feces 21.0 +/- 1.7%). Thus, most oxalate that does enter the circulation is promptly excreted in rats, as in humans. Consequently, even after a large, persistent oxalate challenge, very little oxalate had accumulated in the internal organs, muscle, and skeleton. Unlike humans, however, rats excrete a significant fraction of oxalate in the feces.