We have proposed that the naturally occurring alkaloid ryanodine reduces the release of calcium from the sarcoplasmic reticulum (SR) in cardiac muscle cells. We summarize the data that support this hypothesis and discuss possible mechanisms for 1) the differences in sensitivity to ryanodine displayed by intact skeletal and cardiac muscle preparations vs. that of skinned cardiac cells and isolated SR membranes, 2) the ability of ryanodine to cause either an increase or a decrease in calcium accumulation by isolated skeletal muscle SR vesicles depending on experimental conditions, and 3) the positive inotropic effects produced by ryanodine in cardiac muscle preparations under certain experimental circumstances. In addition, we also show how ryanodine can be used to evaluate the contributions made by SR calcium release to cellular events in striated muscle.