"Postextrasystolic potentiation" of isotonic contractions of the intact isolated rat heart was studied. It was found that the Frank-Starling mechanism does not participate in the increase of the contraction following a premature beat and a compensatory pause. A linear relationship could be demonstrated between the prematurity of the extra beat and the amplitude of the postcompensatory contraction. The increase of amplitude of the postcompensatory contraction was found to counterbalance the decrease of the premature beat. At a constant net rate, whether the rhythm was regular or not, the average contraction height was constant. Postextrasystolic potentiation pattern following an interpolating premature beat was different from postextrasystolic potentiation following a premature beat and a compensatory pause. Postextrasystolic potentiation following an interpolating premature beat could not be distinguished from poststimulation potentiation. It was therefore concluded that postextrasystolic potentiation does not exist as a specific phenomenon.