We used RNA interference, which causes sequence-specific degradation of target mRNAs to suppress the production of parathyroid hormone by cells of patients with secondary hyperparathyroidism in vitro and in vivo. Transfection of small interfering RNA (siRNA) against human parathyroid hormone into monolayers of parathyroid cells cultured from these patients caused a dose-dependent decrease of secretion and mRNA levels with 80% or more suppression using 40 nM siRNA. Parathyroid cells cultured on non-adherent plastic produced spheroid cell aggregates which secreted parathyroid hormone for more than 150 days. Transfection of these spheroids with 50 nM targeted siRNA decreased parathyroid hormone production to 20% of the control level, with half of them being suppressed for 50 days. When parathyroid cells were transplanted into the livers of athymic nude mice, plasma human parathyroid hormone rose to 100-300 pg/ml within one month and remained at about this level for at least 39 days. Systemic delivery of hormone-targeted siRNA into these mice caused a dose-dependent suppression of circulating human parathyroid hormone for at least one month, with a maximum 80% suppression achieved by 80 microg of siRNA. Our study shows that hormone secretion by parathyroid cells of patients with secondary hyperparathyriodism can be suppressed both in vitro and in vivo by targeted siRNAs.