The influence of ejection pressure (EP) on the left ventricular end-diastolic pressure-volume (P-V) relation was examined in 24 paced, isolated canine hearts. A pressure servo system was used to control EP and monitor ventricular volume. For any given contractile state EP was varied: a) by 10- to 20-mmHg increments and the steady-state P-V response observed at each EP level (method A); or b) by instantaneously attaining the isovolumic condition (PO) from a given EDV and examining the immediate P-V response (method B). With either method it was possible to alter the P-V relation; the maximum variation occurred when, for a given EDV or EDP, EP was raised from a value less than 50% of its corresponding peak isovolumic pressure to PO. For the EDP range 2-25 mmHg, the direction and total magnitude of change were: 1) an increase in EDV 1.3-9 ml was required to maintain EDP constant; and 2) for a constant EDV, EDP decreased an average 16.5%+/-1.2 SE (6-23.9%; P less than .02). The EDP decline observed with method B was immediate and reversible within several beats upon returning to the ejecting mode. Thus, under the conditions of this experiment the diastolic pressure-volume relation is a physiological variable dependent on ejection pressure.