Isometrically contracting cat papillary muscles were studied. Muscle length was changed during diastole and returned to control just before the next contraction such that developed force was always measured at the same length. When the diastolic length was increased from a control length, systolic force at the control length increased slowly over several minutes. When the muscle was then held at the increased length, there was an immediate increase in systolic force followed by a small secondary slow increase. Conversely, a decrease in diastolic length from a control length resulted in a slow decrease in systolic force at the control length. When the muscle was then held at the decreased length there was an immediate decrease in systolic force followed by a small secondary decrease. No change in the time course of contraction accompanied the slow force changes after a maintained change of length or a change of diastolic length alone. The magnitude of the slow change of force was proportional to the duration of time in each diastole for which the length was altered and independent of the onset time of a given duration of diastolic length change. The contractility changes were not linearly related to the amplitude of the diastolic length changes. The potentiating effect of a given stretch was greater than the depotentiating effect of a similar release. The development of inotropic changes as a result of diastolic length changes occurred whether or not the muscle was stimulated during the period of the length changes.