Effects of temperature, Ca2+, and ATP on the extent and characteristics of the medium Pi in equilibrium HOH exchange catalyzed by sarcoplasmic reticulum ATPase are reported. Measurements of the patterns of [18O]Pi species formed from highly labeled [18O]Pi show that a single catalytic pathway is involved in the rapid medium Pi in equilibrium HOH exchange with Mg2+ present and the much slower exchange with both Mg2+ and Ca2+ present. A continued high rate of exchange is observed when ATP concentration is increased up to 5 mM even though the amount of phosphoenzyme formed from Pi is much greater at lower ATP concentrations. This result reveals that binding of ATP in some manner causes a pronounced increase in the rate constant for hydrolysis of the phosphoenzyme. During the rapid oxygen exchange in the absence of Ca2+ at 10 and 30 degrees C, the rate of Pi release from the enzyme-Pi complex is about 5-6 times greater than the rate of phosphoenzyme formation. Both Pi release and phosphoenzyme formation are much slower in the presence of Ca2+, with a greater relative tendency for phosphoenzyme formation, particularly at the lower temperature.