Normal blood levels of potassium are critical for maintaining normal heart electrical rhythm. Both low blood potassium levels (hypokalemia) and high blood potassium levels (hyperkalemia) can lead to abnormal heart rhythms. The aim of the work presented is to study the effect of potassium concentration on the excitation wave in the cardiac tissue. Results have been obtained both in the experimental model, which is a monolayer of neonatal rat cardiomyocytes, and in the modified Korhonen computer model, designed for ventricular rat neonatal cardiomyocytes. The existence of non-sodium excitation waves under a strong hyperkalemia (more than 10 mM K + in the extracellular environment) in the cardiomyocyte monolayer has been found and has also be confirmed by inactivation of sodium channels with a specific channel blocker.