It is not known whether ventricular relaxation can be altered by ischemia independent of associated hemodynamic changes. The effect of acute myocardial ischemia on diastolic relaxation was studied under controlled hemodynamic conditions in 16 anesthetized dogs on right-heart bypass. In nine dogs, left ventricular end-diastolic pressure was maintained constant throughout the experiments. Ischemia produced a significant prolongation of the isovolumetric relaxation period (IVRP) from 29 +/- 3 (SE) to 88 +/- 7 ms (P less than 0.01). Ischemia, per se, the associated decrease in contractility, or the fall in peak left ventricular pressure (LVP) may have contributed to the increase in IVRP. The latter was not the only mechanism involved, because in seven dogs studied with constant peak LVP, IVRP again was prolonged from 64 +/- 8 to 95 +/- 8 ms (P less than 0.01). Moreover, in five nonischemic hearts in which peak LVP was maintained constant, contractility was decreased by sodium pentobarbital to the same extent as with ischemia; IVRP did not change. Thus, the additional prolongation of the IVRP in the ischemia experiments is secondary to a direct effect of ischemia on the relaxation process of the myocardium.