Caffeine has been known to induce the contraction of rabbit aortic ring resulting from Ca²⁺ release from the intracellular stores. But in contrast, contraction of aortic ring induced by depolarizing agents or agonist was reported to be suppressed by caffeine. The present study was intended to examine the effect of caffeine on Ca²⁺ movement across the plasma membrane and actomyosin ATPase activity of vascular smooth muscle to elucidate the modes of action of caffeine on the vascular smooth muscle. Aortic ring preparation were made from the rabbit thoracic aorta and the endothelial cells were removed from the ring by gentle rubbing. The contractilty of the aortic ring was measured under varying conditions, and Ca²⁺ influx across the membranes of the aortic ring was measured with Ca²⁺ sensitive electrode with and without caffeine and the effect of caffeine on actomyosin ATPase activity were measured by modified Hartshrone's method. ⁴⁵Ca wash out curves with and without caffeine were studied by Richard's method. The results were summarized as follows: 1) Caffeine inhibited the contractilty induced by norepinephrine. high K⁺, and histamine. but caffeine alone induced a transient contraction of vascular smooth muscle. The caffeine induced contraction was demonstrable even in the absence of external Ca²⁺. 2) Caffeine increased ⁴⁵Ca efflux from vascular smooth muscle. 3) In the presence of propranolol, the inhibitory effect of caffeine on epinephrine induced contraction still persisted. 4) Caffeine decreased norepinephrine induced Ca²⁺ influx through the plasma membranes of aortic ring. 5) Caffeine decreased the actomyosin ATPase activity of vascular smooth muscle. From the above results, it is suggested that caffeine induces the contraction of vascular smooth muscle by release of Ca²⁺ from intracellular Ca²⁺ stone, but inhibits drug-induced contraction by decrease of Ca²⁺ influx across the plasma membranes and a decreased Ca²⁺ sensitivity of contractile protein in vascular smooth muscle.