It is now generally accepted that the contraction of skeletal myofibrils, actomyosin, and glycerinated muscle fibers requires the presence of Ca++ in addition to Mgi+ and adenosine triphosphate (2-7). The relaxing effect of chelating agents (8-10) and skeletal muscle microsomesl (11-16) is explained by their ability to lower the Ca++ concentration of the test system below lop6 M in the presence of 5 mM adenosine triphosphate and 5 m&l MgC $. The skeletal muscle microsomes accomplish this by a remarkably powerful uphill transport system for Ca+ f, which operates in the presence of Mgf+ and ATP. The actual mechanism of the transport process is not known. According to Ebashi (14-16), the Ca+ f uptake is the reflection of Ca++ binding to specific receptor sites made accessible for Ca++ by ATP and Mg. The binding of 1 mole of ATP for about 70 moles of Ca+ f has been demonstrated (16). Hasselbach and Makinose (11-13) showed that the Ca+ f uptake is greatly augmented in the presence of oxalate, and they ascribed the Ca++ uptake to an active transport process which derives its energy from the hydrolysis of ATP. Distinction between the two alternative explanations can be made if the state of Ca++ within the microsomes is known. For this purpose, the rate of Ca+ f uptake and release has been measured in the presence and absence of potentiators (oxalate, pyrophosphate) and inhibitors.