Preincubation of sarcoplasmic reticulum (SR) membranes with a combination of ATP and NaF resulted in inhibition of Ca2+ accumulation and stimulation of Ca2+-ATPase and Ca2+ efflux. Under the same conditions, the activity of the SR phosphoprotein phosphatase was inhibited and the phosphorylation of two polypeptides with apparent molecular masses of 160 and 150 kDa was obtained. The effect of ATP is specific, since the ATP analogue adenosine 5'[βγ-imino]triphosphate did not replace for ATP. In the absence of NaF, ATP was ineffective. The phosphorylation of the 160 kDa and/or 150 kDa proteins and the stimulation of Ca2+ efflux are clearly related. The phosphorylation of both proteins and the increase in Ca2+ efflux show a similar dependence on the concentration of ATP. The level of protein phosphorylation and the stimulation of Ca2+ efflux were also controlled by the NaF concentration which inhibits the phosphoprotein phosphatase. Similar concentrations of NaF were required for the inhibition of phosphoprotein phosphatase and of net Ca2+ accumulation, as well as for the stimulation of phosphorylation of both polypeptides. Quantitative analysis revealed a linear correlation between these three activities. Dicyclohexylcarbodi-imide, which inhibited Ca2+ efflux, also inhibited the phosphorylation of the two polypeptides. These results suggest the involvement of the phosphorylation/dephosphorylation of 160 kDa and/or 150 kDa polypeptides in the activation of Ca2+ release from SR membranes.