High affinity Ca2(+)-activated, Mg2(+)-dependent ATP-ase [Ca2+ + Mg2+)-ATPase) activities were characterized in the membrane fractions of the porcine aorta. The (Ca2+ + Mg2+)-ATPase activity, similar to those found in the plasma membranes of the erythrocyte and the heart, ie, Ca2(+)-pumping ATPase activity, was not found in the membrane fractions isolated by the conventional method. The activity, however, became apparent in a plasma membrane-enriched fraction obtained from the microsomes treated with digitonin. The enzyme activity was stimulated by a purified C-kinase and by cyclic GMP or a purified cyclic GMP-dependent protein kinase (G-kinase). In addition to the Ca2(+)-pumping ATPase which requires millimolar concentration of Mg2+ for its activity, another high affinity (Ca2+ + Mg2+)-ATPase was detected that required micromolar concentration of Mg2+ for its full activation. Cell fractionation studies suggested its localization to plasma membranes, but the biochemical characteristics of the enzyme indicated that the enzyme could not be a biochemical expression of the plasma membrane Ca2+ pump.