T3 administration to rats exerts quite different effects on enzyme activities associated to liver microsomal membranes such as G-6-Pase, Mg ATPase and Ca2(+)-dependent ATPase: in fact G-6-Pase activity is significantly enhanced, Mg ATPase is not affected whereas Ca2(+)-dependent ATPase is drastically inhibited. The T3 induced decrease in Ca2(+)-dependent ATPase activity is associated with a net reduction (to about 50% with respect to controls) of the Ca2+ sequestration in liver microsomal vesicles. The enhanced level of inorganic phosphate in the endoplasmic reticulum due to the stimulation of G-6-Pase activity does not significantly affect the uptake of calcium in microsomal vesicles. The decreased Ca2(+)-dependent ATPase activity is associated to an enhanced level of the enzyme in the phosphorylated form (E-P). This suggests that in liver preparations from T3 treated rats the turnover of ATP and cleavage of E-P is reduced, thus resulting in the accumulation of the phosphorylated intermediate. The accumulation of E-P is in agreement with the inhibition of the calcium sequestration since the active transport of this cation in microsomal membranes requires the hydrolysis of the E-P complex.