A measurable factor determining the filling of the heart is the opposition that the myocardium offers to being stretched. This is expressed as “renitence,” which is the ratio of filling pressure to the rate of inflow. Isolated frog ventricle was connected to a vessel containing Ringer's solution at a fixed head of pressure. Change in the volume of the heart was recorded optically. From these time-volume curves, renitence was calculated.For the frog ventricle filling and emptying against a fixed head of pressure, the renitence during the filling period first decreases, passes through a minimum, and then increases. The minimum and average renitences occurring during a cycle appeared to be increased by cooling, a rise in filling pressure, pentobarbital or a combination of hypoxia and acidity.The renitence early during the filling cycle is largely determined by the residual tension from the previous systole, and the subsequent increase seems to be determined by the elastic properties of the myocardium.