Potential protective effects of aerobic exercise training on the myocardium, prior to an ischemic event, are not completely understood. The purpose of the study was to investigate the effects of exercise training on contractile function following ischemia/reperfusion (Langendorff preparation with 15 min global ischemia/30 min reperfusion). Trabeculae were isolated from the left ventricles of both sedentary control and 10-12 week treadmill exercise trained rats. The maximal normalized isometric force (force/cross-sectional area; P₀/CSA) and shortening velocity (V₀) in isolated, skinned ventricular trabeculae were measured using the slack test. Ischemia/reperfusion induced significant contractile dysfunction in hearts from both sedentary and trained animals; left ventricular developed pressure (LVDP) and maximal rates of pressure development and relaxation (±dP/dt) decreased, while end-diastolic pressure (EDP) increased. However, this dysfunction (as expressed as percent change from the last 5 min before ischemia) was attenuated in trained myocardium (LVDP, sedentary -60.8 ± 6.4%; 32.0 ± 5.5 mmHg), vs. trained -15.6 ± 8.6%; 64.9 ± 6.6 mmHg); +dP/dt_max, sedentary -54.1 ± 4.7%; 1058.7 ± 124.2 mmHg/s), vs. trained -16.7± 8.4%; 1931.9 ± 188.3 mmHg/s); -dP/dt_max, sedentary -44.4 ± 2.5%; -829.3 ± 52.0 mmHg/s), vs. trained -17.9± 7.2%; -1341.3 ± 142.8 mmHg/s); EDP, sedentary 539.5 ± 147.6%; 41.3 ± 6.0 mmHg), vs. trained 71.6 ± 30.6%; 11.4 ± 1.2 mmHg). There was an average 26% increase in P₀/CSA in trained trabeculae compared to sedentary controls and this increase was not affected by ischemia/reperfusion. Ischemia/reperfusion reduced V0 by 39% in both control and trained trabeculae. The relative amount of the beta isoform of myosin heavy chain (MHC-) was two-fold greater in trained trabeculae as well as in the ventricular free walls. Despite a possible increase in the economy in the trained heart, presumed from a greater amount Exercise training and ischemia/reperfusion injury of MHC-, ischemia/reperfusion reduced V₀, to a similar extent in both control and trained animals. Nevertheless, the trained myocardium appears to have a greater maximum force generating ability that may, at least partially, compensate for reduced contractile function induced by a brief period of ischemia.