The effects of monensin (10 µmol/L) alone and in presence of verapamil (3 µmol/L) or ouabuin (1 µmol/L) or glibenclamide (10 µmol/L) or BaCl₂ separately were studied on the electrophysiological properties represented by force of contraction, duration of action potential, and membrane resting potential of isolated atrial and ventricular muscle of guinea –pig heart. Monensin induced a transient increase in the force of contraction of atrial and ventricular muscles. It caused a decrease in action potential duration of papillary muscle, this effect was reversed by glibenclamide. Monensin increased the action potential duration of atrial muscle in absence or presence of glibenclamide. In presence of ouabain monensin induced a significant depolarization of resting membrane potential in both atrial and ventricular muscles. Verapanil did not antagonize the effects monensin. Monensin induced an increase in Na⁺ a transient increase in Ca²⁺ and a decrease in K⁺ contents of atrial and ventricular tissues. From the data of the present work it could be concluded that, Na⁺ pumped out the cell by Na⁺/K⁺ pump was less in atria then in ventricles in presence of moninsin. Less consumption of intracellular ATP in ventricular cells than in atrial cells, thus the ATP of ventricular cells did not reach the critical concentration level which open ATP dependent K⁺ channels, The study also showed absence of Na⁺ dependent K⁺ channels in guinea-pig atria.