The late phase of repolarization of human ventricular cardiac myocytes mainly depends on the rapid and on the slow component of the delayed rectifying K⁺ current (IK_r and IK_s). In contrast to rodents (e.g. mice and rats), in guinea pig ventricular myocytes the final repolarization is also based on IK_rand IK_s suggesting that guinea pigs may very well be suited as experimental models for investigations of late repolarization. However, the physiological regulation of repolarization has not been characterized in these animals previously. The aim of this study, therefore, was to describe the heart rate (HR) dependent regulation of ventricular repolarization, assessed by analysis of the QT interval in freely moving guinea pigs. ECG and physical activity were recorded in guinea pigs (n=12, 300- 390 g) using telemetric ECG devices. ECGs of different HR ranges and of different activity levels were analyzed using a semi automatic software. The QT interval was derived following two definitions of the QT interval (QT_end and QT_peak). In total, 23.808 ECG complexes spanning a HR range from 90 to 415 bpm could be analyzed successfully using both QT definitions. The QT interval displayed a steep HR dependency with QT_end ranging from 197 to 80 ms and QT_peak from 162 to 68 ms. These values could well be described by linear regressions (r²=0.81) with a slope of -0.37 (QT_end) and -0.35 (QT_peak), respectively. In summary, the ECG of freely moving guinea pigs can be retrieved using telemetric devices and ECG parameters, including the QT interval as a marker for ventricular repolarization, can readily be analyzed. Very similar to humans, the ventricular repolarization of guinea pig hearts strongly depends on the prevailing heart rate with a linear relationship. Thus, the present study supports the notion that the guinea pig may represent a valuable animal model for telemetric ECG studies relevant to the human cardiac physiology.