The mechanical behavior of isolated cat, rat, and dog ventricular muscle was examined during hypoxia and after reoxygenation. During hypoxia, an early abbreviation of tension duration was followed by a decline in the rate of tension development. After reoxygenation, a marked, early prolongation of tension development and relaxation time was invariably observed with little, if any, increase in peak tension. As recovery progressed, the duration of contraction gradually shortened as tension returned to control levels. This phenomenon was also observed in the intact dog heart after release of a coronary artery ligature. Isometric tension gauges sewn to ischemic portions of the left ventricle demonstrated that after reinstitution of coronary flow, segment tension duration “outlasts” the duration of left ventricular pressure development and is associated with ventricular irritability. Epicardial electrograms showed shortening of the QT interval within the ischemic segment with prolongation of the QT interval after release of the coronary ligature. Prolongation of tension development during recovery from hypoxia was not observed in experiments with rat skeletal muscle. These observations identify localized mechanical abnormalities during recovery from myocardial ischemia which may be important in the syndrome of acute coronary heart disease.