Increased oxidative stress resulting from enhanced production of reactive oxygen species and/or inadequate mechanisms of antioxidant defenses has been recognized as an important factor contributing to the initiation and progression of cardiac dysfunction under a wide variety of pathophysiological conditions. The main objective of this study was to examine the effect of electrically induced tachycardia on oxidative stress and the capacity of antioxidant defenses in the normal and hypertrophied left ventricle (LV) in the rat. Left ventricular hypertrophy (LVH) was produced by banding the descending abdominal aorta. The activities of antioxidant enzymes, concentrations of non-enzymatic antioxidants, and biomarkers of oxidative stress were measured in the LV of aortic-banded animals (LVH), untreated or banded rats subjected to short-term (45 min) atrial pacing [(CTR + S) and (LVH + S), respectively], and untreated (CTR) or sham-operated (SHAM) controls. The results indicate that the increase in heart rate in vivo as a result of atrial pacing to a maximum level, independent of sympathetic nerve activity, leads to a substantial increase in oxidative stress and a marked decline in the activities of antioxidant enzymes in both the normal and hypertrophied left ventricle of the rat. The accompanying increase in tissue content of alpha- and gamma-tocopherols seem to contribute to attenuation of the oxidant stress-related loss of thiol stores in the LV. Stable left ventricular hypertrophy induced by aortic banding for six weeks has a minor impact on the capacity of the endogenous antioxidant defense system in the LV, but significantly and negatively affects the ability of the heart LV to tolerate the stress of tachycardia.