Doxorubicin (DOX) is the most common anticancer agent prescribed to treat childhood cancers. Utilization of DOX is limited by a dose-dependent cardiotoxicity that can eventually progress to heart failure. It has been suggested that children are more susceptible to develop chronic heart failure than adults. However, the majority of DOX studies were focused on acute cardiotoxicity in adult animals. Additionally, the exact cumulative dose of DOX that would cause chronic cardiac dysfunction in juvenile rats was undetermined. Recent studies have reported exercise training attenuated DOX-induced acute cardiotoxicity in adult rats. Therefore, the purpose of this study was to determine whether exercise rehabilitation could attenuate DOX-induced cardiac dysfunction in juvenile rats. To better understand the mechanisms that may cause chronic cardiac dysfunction, myosin heavy chain (MHC) distribution and sarco-(endo)-plasmic reticulum calcium-ATP_ase-2a (SERCA2a) protein expression were examined. Twenty-five-day old male Sprague Dawley pups were randomly assigned to either voluntary wheeling (WR) or sedentary (SED) groups. Immediately following the exercise assignment, rats were randomly assigned to receive either saline (WR+SAL and SED+SAL) or DOX (WR+SAL and WR+DOX). Rats in the DOX groups received i.p. injections of DOX (2mg/kg/day) for 7 consecutive days (14mg/kg cumulative). Rats assigned to SAL groups received an equivalent volume of SAL. In vivo cardiac function was monitored weekly using echocardiography to determine if cardiac dysfunction was present. Nine weeks after completed DOX treatment, cardiac function was assessed, and left ventricular tissue was analyzed for MHC isoform distribution and SERCA2a protein expression. The SED+DOX rats had significant in vivo diastolic dysfunction and ex vivo diastolic and systolic dysfunction when compared to SED+SAL animals However, no in vivo or ex vivo cardiac dysfunction was found in WR+DOX animals, suggesting that exercise prevented DOX-induced cardiac dysfunction. β-MHC isoform distribution and SERCA2a expression of SED+DOX rats were not different from SED+SAL rats, suggesting that SERCA2a andβ3-MIIC isoform distribution may not contribute to DOX-induced diastolic and systolic dysfunction or the cardioprotective mechanisms of exercise in the juvenile rat model.