Cardiac ischemia is followed by the potassium deficiency in cardiomyocyte [1-2]. The intracellular potassium concentration is an integrative result of active influx through Na+/K+-ATPase and passive transport via K+ channels and anti- and symporters. Therefore the intensive potassium release may mask the pump activity. As Rb+ is assumed to be a biological congener of K+, and extracellular medium is modified to contain rubidium ions, 87Rb NMR have been developed to mark K+ fluxes between extracellular space and tissue [3]. Electron Probe Microanalysis (EPMA) allows the determination of elemental contents in cellular cytoplasm and this was utilized for Rb analysis in the neural and cardiac cells [4-5]. We applied EPMA to compare the kinetic parameters of potassium influxes at normoxic and ischemic conditions. After decapitation the heart from 300-350 g male Wistar rat was perfused in the Langendorff mode with Tyrode’s solution at a constant pressure of 70 mm Hg and the myocardial temperature was maintained at 38°C. Normoxic conditions were created with 95% O2 + 5% CO2 aeration of Tyrode. Ischemia was employed using glucose free solution equilibrated with 100% argon. Following 6 minutes of equilibration, hearts were perfused 5 min, 30 min, 45 min and 60 min at normoxic or ischemic conditions. Then papillary muscles were excised from the heart for EPMA technique [1]. Freshly isolated papillary muscle was frozen immediately by plunging into liquid propane (90°K) and cut in cryostat (240°K). Tissue cryosections were picked up and freeze-dried for 12 hours in a high vacuum. Freeze-dried cryosections were mounted on aluminium grids and intracellular rubidium content was examined in a scanning electron microscope JEOL JSM-U3 equipped with wavelength dispersive spectrometers. The section was viewed in the STEM mode at an accelerating voltage of 25 kV, a beam current of 1-5 nA and a probe diameter about 0.1 μm. EPMA data obtained are summarised in Table 1. Experimental data of Rb EPMA allowed us to estimate the kinetic parameters [5]. Thus EPMA provides a method of determining uni-directional K fluxes in the cardiac myocell of perfused rat heart.