In order to clarify the Ca2+ mediated regulatory mechanism of vascular smooth muscle, the correlation between Ca2+ movements in the cell and mechanical response was investigated using isolated rat aorta and bovine basilar artery. K+ induced contraction of vascular smooth muscle required Ca2+ in the medium, and the contraction was always associated with a large increase in Ca2+ influx. In the absence of external Ca2+, norepinephrine (NE) or serotonin (5HT) induced rapid contraction of the aorta and basilar artery, respectively. Caffeine (20 mM) also induced tension in the absence of Ca2+. The results obtained in these experiments suggest that at least four kinds of calcium exist in bound from in the cell, i. e., (1) loosely and (2) tightly bound calcium on the external surface of the cell membrane, (3) calcium localized in the internal surface of the cell membrane and (4) calcium stored in the intracellular store site, such as the sarcoplasmic reticulum. The intracellulary stored calcium, both (3) and (4), which can be mobilized by the action of certain drugs (NE, 5HT or caffeine), probably plays an important role for the regulation of the mechanical activity of vascular smooth muscle.