A membrane fraction enriched in endoplasmic reticulum was prepared from rat parotid glands by using sucrose-gradient centrifugation. The fraction showed a 10-fold increase in specific activity of NADPH: cytochrome c reductase activity over that of tissue homogenates and minimal contamination with plasma membranes or mitochondria. The endoplasmic reticulum fraction possessed both Mg2+ -stimulated ATPase as well as Ca2+, Mg2+-ATPase [( Ca2+ + Mg2+)-stimulated ATPase]activity. The Ca2+, Mg2+-ATPase required 2-5 mM-Mg2+ for optimal activity and was stimulated by submicromolar concentrations of free Ca2+. The Km for free Ca2+ was 0.55 microM and the average Vmax. was 60 nmol/min per mg of protein. The Km for ATP was 0.11 mM. Other nucleotides, such as GTP, CTP or ADP, could not substitute for ATP in supporting the Ca2+-activated nucleotidase activity. Increasing the K+ concentration from 0 to 100 mM caused a 2-fold activation of the Ca2+, Mg2+-ATPase. Trifluoperazine, W7 [N-(6-aminohexyl)-5-chloronaphthalene-1-sulphonamide] and vanadate inhibited the enzyme. The concentration of trifluoperazine and vanadate required for 50% inhibition of the ATPase were 52 microM and 28 microM respectively. Calmodulin, cyclic AMP, cyclic AMP-dependent protein kinase and inositol 1,4,5-trisphosphate had no effect on the ATPase. The properties of the Ca2+, Mg2+ -ATPase were distinct from those of the Mg2+-ATPase, but comparable with those reported for the parotid endoplasmic-reticulum Ca2+-transport system [Kanagasuntheram & Teo (1982) Biochem. J. 208, 789-794]. The results suggest that the Ca2+, Mg2+-ATPase is responsible for driving the ATP-dependent Ca2+ accumulation by this membrane.