Intensive constitutive production of nitric oxide (NO) during physical training improves vasodilatation and heart function. However, it remains unclear how NO takes part in myocardial adaptation to workload, which is accompanied by an increased heart inflow and intracellular calcium content. Using isolated rat heart by Langendorf preparation, we studied myocardial response to gradually increased left ventricular volume (Frank-Starling low) and increasing concentration of Ca2+ in the perfu-sion solution (from 1.7mM to 12,5 mM) in trained and un­trained rats. It was shown that 4 weeks swimming course improved heart function: heart rate was decreased; contractile activity (dP/dt max) and coronary flow were increased by 20% and 33%, respectively. Equal volume stretching of bal­loon in left ventricle provoked greater contraction in trained comparing to untrained hearts, demonstrating extended func­tional reserves after swimming course. Mitochondrial membrane potential was significantly increased in hearts of trained rats. Furthermore, training prevented fast increase of the end dias-tolic pressure during calcium upload. Mitochondrial factor release due to opening of mitochondrial permeability transi­tion pore (MPTP) in trained hearts was detected at higherconcentrations of calcium that reveals extended calcium capacity of mitochondria and lesser sensitivity of MPTP to its induc­tor – calcium. Blockade of NO synthesis with L-NAME ap­plication of (10-4 M for 15 min) abolished reaction of trained heart during Frank-Starling and calcium upload. Thus, heart adaptation to physical training and extension of functional reserves in heart are provided by endogenous NO production.