Only one mathematical model of the excitation-contraction coupling (ECC) in the rat cardiomyocytes, the Pandit–Hinch–Niederer model (PHN) seems published so far (see e.g. Niederer and Smith; BJ, 2007). Simulation of the slow force response after the myocardium stretch was significant achievement of the PHN. For this the authors had to introduce additionally stretch-activated channels in the Pandit-model of the Action Potential (AP) used as electrophysiological module of the PHN, while such channels were not accounted for in the original Pandit-model (Pandit ea; BJ, 2001). Significant limitation of the PHN is incapacity to simulate quick effects of the length changes, i.e. Mechano-Calcium and Mechano-Electric Feedback effects (MCMEF) manifesting themselves during the twitches of cardiac muscles (trabeculae, papillary muscles) in a number of important phenomena, including the load-dependence of relaxation. We present here a new model of ECC in the rat cardiomyocyte where we combine the Pandit-model and Hinch-model of calcium handling (Hinch ea; BJ, 2004) with the module of the mechanical activity early developed for the 'Ekaterinburg-Oxford' mathematical model (EO-model) (Sulman ea; BMB, 2008). EO-model describes ECC in intact cardiomyocytes for some species of laboratory animals (e. g. rabbit, guinea pig), being parametrically adapted to each particular one. However, EO-model cannot be applied to simulate adequately the rat AP due to the very specific features of the latter. Inclusion of the Pandit-model of the rat AP is a way to overcome this drawback. Mechanisms of cooperativity of regulatory and contractile proteins is a key link of our module of the mechanical activity that earlier allowed us to simulate within the EO-model a wide range of the MCMEF effects in myocardium. In our new model of the rat cardiomyocytes these effects (including the load-dependence of relaxation) also turn out to be reproduced and result from the cooperativity mechanisms.