N-(3-[2-Chlorophenyl]propyl)-(R)-α-methyl-3-methoxybenzylamine (NPS R-568) is an orally active compound that activates Ca²⁺ receptors on parathyroid cells and rapidly suppresses plasma levels of parathyroid hormone (PTH) and Ca²⁺(ED₅₀, 1 and 10 mg/kg, respectively). We now show that increased calcitonin secretion contributes to NPS R-568-induced hypocalcemia. In parathyroidectomized thyroid-intact rats in which normocalcemia was restored by PTH infusion, NPS R-568 rapidly reduced plasma Ca²⁺ levels, indicating that decreased PTH secretion was not solely responsible for the hypocalcemia seen in normal animals. NPS R-568 decreased plasma Ca²⁺ levels in thyroidectomized parathyroid-intact rats, but the rate of onset of hypocalcemia was slower than in controls. In contrast, NPS R-568 had no effect on plasma Ca²⁺ levels in PTH-infused, thyroparathyroidectomized rats, providing evidence that increased calcitonin secretion caused the hypocalcemia in PTH-infused parathyroidectomized rats. NPS R-568 rapidly increased plasma calcitonin levels to a peak at 10 to 20 min after oral dosing (ED₅₀ 40 mg/kg). NPS R-568 did not affect the rate of disappearance of ⁴⁵Ca from blood, indicating that hypocalcemia resulted from decreased influx of Ca²⁺into the circulation and not from increased efflux. This suggests that NPS R-568-induced hypocalcemia resulted solely from reduced efflux of Ca²⁺ from bone after increased calcitonin and reduced PTH secretion. Thus, NPS R-568 causes hypocalcemia by activating Ca²⁺ receptors on C cells and parathyroid cells; however, NPS R-568 is about 40 times more potent in reducing PTH levels than in increasing calcitonin levels.