A plasma membrane-enriched fraction of vesicles from rat mesenteric artery possessed ATP-dependent calcium properties similar to those which have been reported previously. The catalytic subunit from a soluble, cyclic AMP-dependent protein kinase was prepared from the same tissue, in partially purified form. Calcium uptake by membrane vesicles increased 30% in the presence of this kinase. This effect was proportional to the kinase concentration, and was blocked by either catalytic subunit inhibitor or by prior boiling of the subunit preparation. Stimulation of calcium uptake by the kinase occurred maximally between 1 and 10 µ Mfree calcium. These findings suggest that cyclic AMP and its dependent protein kinase may participate in the regulation of smooth muscle tension at high free myoplasmic calcium concentrations.