- Abstract Thyroid hormones inhibit Ca 2+ accumulation and ATPase activity of isolated sarcoplasmic reticulum vesicles. Half-maximal inhibition was obtained by about 2.5 μM. The ATP hydrolysis activity of the purified (Ca 2+ + Mg 2+ )-ATPase or the SR vesicles, in the presence of the Ca 2+ ionophore A23187, is not inhibited by T 3 or T 4 . Modification of T 3 or T 4 in the ring portion, but not in the amino portion, of te molecules results in T 4 and T 3 analogues which are unable to inhibit Ca 2+ accumulation. T 3 and T 4 have no significant effect on various partial reactions of the transport cycle such as: the binding of ATP and Ca 2+ , or ADP-ATP exchange and E-P formation from ATP, but they inhibit the E-P formation from inorganic phosphate (P i ) and ATP-P i exchange. The inhibition of both Ca 2+ accumulation and ATPaseactivity by T 3 or T 4 is increased in the presence of P i . Binding sites for [ 125 I]T 3 and for [ 125 I]T 4 in SR proteins were demonstrated using either equilibrium dialysis or gel overlay techniques. The results suggest that the thyroid hormones inhibit the ATP-dependent Ca 2+ accumulation, probably by inhibiting the transport of anions which act as the Ca 2+ precipitating anion.