The contractile performance of isolated myocardium is critically dependent on temperature. From 32 to 16 C, both diastolic and systolic force as well as contraction duration increase exponentially. Diastolic force approached thermal insensitivity when bathing medium calcium was removed. Diastolic force predicts by simple linear functions the entire profile of the ensuing contraction, including systolic force, rates of force development, and contraction duration. These findings are suggestive of an underlying contractile process common to both the diastolic and systolic states of the muscle.