Previous studies have shown that myocardial ischemia causes a reduction in the number of of cardiac ryanodine receptors. However, how ischemic preconditioning(IP) affects the density of ryanodine receptors remains uncertain. To clarify the effect of IP on ryanodine receptors, the density of high-affinity 3H-ryanodine binding sites and thedissociation constant for ryanodine of the myocardium were examined in anesthetized open chest dogs with or without IP. Group S(n=6)