The effect of adenosine was studied on the efflux and uptake of 45Ca by canine coronary arteries (O.D. less than 1 mm) using standard isotope flux techniques. Adenosine (10(-4) M) did not reduce the uptake of 45Ca by coronary arteries incubated in normal physiological solution and had an insignificant effect on the increased 45Ca uptake due to high [K+]0. However, adenosine (10(-4) M) significantly (P less than 0.001) inhibited the elevated 45Ca efflux that was induced by K+ depolarization (150 mM) and abolished the transient rise in efflux rate produced by aminophylline (10(-3) M). Aminophylline had no significant effect on the uptake of 45Ca. This study suggests that the vasodilator action of adenosine may be related to a reduction of release or an increased binding of calcium which is superficially stored. It also indicates that the antagonism between adenosine and aminophylline in the coronary blood vessels may not involve calcium influx.