Heart rate dependency of ventricular repolarization (VR) parameters has been a well known phenomenon for many years. Other contributing factors, such as Mechano-electric Coupling, are still under investigation. In this work, we analyze step response of VR parameters after abrupt change in mechanical end-diastolic load. 10 isolated rabbit hearts were perfused in Working Heart mode. The protocol consisted of abrupt changes in preload level of 3 cm/fcO. Left ventricle electric activity was recorded by seven unipolar leads at sampling frequency 10 kHz. A second-order transfer function model was used to identify basic parameters of Q.TEND and QTPEAK step responses, detected in reconstructed global pseudo-lead. Results show inverse relationship between changes in the preload and the length of QT related intervals. Increase of the preload causes significant (p<0.05) shortening of both intervals within the range of 1.6-26.9 ms. On the contrary, preload decreasing leads to prolongation of repolarization parameters, however, with significantly lower (p<0.05) average magnitude. Dynamic response is highly individual among each heart. Average time needed to reach stable QT in our data is 336.0 seconds, while min. and max. value is 41.4 and 894.0 seconds, respectively.