In cat papillary muscles contracting physiologically, increasing the afterload caused a biphasic change in contractility. In response to an increase in afterload, contractility (as measured by peak shortening, peak developed force, or peak dF/dt) initially decreased (antihomeometric autoregulation) over the first few beats and then increased slowly with T( 1/2 ) of about 3 min at 30°C and about 1 min at 37°C (homeometric autoregulation). The antihomeometric autoregulation is due to decreased active shortening when the afterload is increased, since it also occurs in response to increased afterload in isotonic contractions. The secondary slow increase in contractility is primarily due to the increase in mean diastolic length that occurs as a result of increased afterload. The time course and the magnitude of the biphasic change in contractility are very similar to those observed in response to afterload increase in intact hearts; we suggest that the secondary slow increase in contractility that we observed is a contributory mechanism to homeometric autoregulation (or the Anrep effect), as it is observed in the whole heart.