To evaluate the effect of thyroid function on the number of Na-K pumps in skeletal muscle, the number of 3H-ouabain binding sites was measured in biopsy specimens from the vastus lateralis muscle of euthyroid subjects and patients with hypothyroidism or hyperthyroidism. In hypothyroidism there was a decrease of 50% in 3H-ouabain binding sites, and in hyperthyroidism there was an increase of 68% in 3H-ouabain binding sites. When thyroid status became normal after treatment the number of 3H-ouabain binding sites also became normal. There were significant correlations between several thyroid function tests and the number of 3H-ouabain binding sites. Free T4-index gave the highest r value (0·87). These changes may account for the variations in digitalis sensitivity associated with thyroid disorders.