Parathyroid cells were isolated from healthy rats, encapsulated in alginate-polylysine membranes, and injected intraperitoneally into rats on which total parathyroidectomies had been performed. Three days posttransplant, serum calcium and PTH-M concentrations had increased to near-normal levels in the recipient animals. Similar results were observed in a separate group of parathyroidectomized rats 3 days after free parathyroid cells were implanted, but within 4 weeks serum calcium and PTH-M concentrations had decreased almost to pretransplant levels in these rats. In the rats with encapsulated cell transplants, by contrast, serum calcium and PTH-M levels were significantly higher, even after 8 weeks. No therapeutic effects were observed in rats injected with empty capsules or in the control group, which received no capsules or cells. These results indicate that transplants of microencapsulated parathyroid cells can temporarily reverse aparathyroidism in rats without the use of immunosuppressive drugs, and that further studies are warranted to investigate possible future clinical applications of this treatment.