The hypothesis that thyroid calorigenesis is mediated by stimulation of active Na⁺ transport was tested by measuring the Q_o2 of liver slices and skeletal muscle (diaphragm) from thyroxine- and triiodothyronine-injected thyroidectomized and normal rats in media fortified with ouabain (10^-3 M) and/or free of Na⁺ or K⁺. In both tissues, more than 90% of the increase in Q_o2 produced by injections of thyroid hormone in euthyroid rats was derived from increased energy utilization by the Na⁺ pump. In triiodothyronine-treated thyroidectomized rats, activation of Na⁺ transport accounted for 90% or more of the increment in Q_o2 in liver and 40% or more of the increment in diaphragm. Intracellular Na⁺, K⁺, and Cl^- concentrations were measured in euthyroid and hyperthyroid liver and diaphragm. The transmembrane Na⁺ and K⁺ concentration differences were significantly increased in both tissues by the administration of triiodothyronine. These results indicate that thyroid hormone activates Na⁺ extrusion and K⁺ accumulation either by increasing the local concentration of ATP or by direct stimulation of the Na⁺ pump.