Using subcellular membrane fractions isolated from smooth muscles of dog mesenteric arteries and veins, we compared their enzymatic activities, Ca2+ bindind and Ca2+ transport. The micro-somal (MIC)and plasma membrane(F2) fractions were similarly enriched 4-5 and 10-12 fold respectively in the plasma membrane marker enzymes over the starting membrane mixture. However, MIC and F2 fractions from venous smooth muscle showed higher Mg2+-ATPase activity, slightly increased Ca2+ binding and reduced Ca2+-transport compared to the corresponding fractions from arteries.