In view of the recent work on isolated cardiac muscle which has shown that situations can occur in which the force and velocity of contraction are chanced in different directions, we have questioned the usefulness of the search for a single index of contractility related to the rate of rise of tension, a trend which dominates studies of the heart in vivo. He have attempted to show, in the heart in vivo, that situations do occur in which the duration of contraction can determine the force of contraction or in. which force and velocity of contraction are changed in different directions such that purely a change in the velocity dependant aspects of contraction would not describe the inotropic intervention.