For the elucidation of the mechanism of inotropic effect of cardiac glycosides the relationship between the inhibitory effect of the drug on Na ion minus K ion-AtPase and the alteration of contractile tension will be studied. For this purpose the alteration of intracellular Na ion concentration and that of contractile tension will be measured simultaneously. The intracellular Na ion concentration will be measured by the Na ion selective microelectrode which is constructed by the Na ion sensitive glass. The microelectrode had an exposed tip length of about 70 mu and a tip to tip length less than 10 mu. It was found that intracellular Na ion activity in sheep Purkinje fibers was 6 approximately 7 nM. Assuming 113 mM Na ion of extracellular fluid, this indicates an equillibrium potential of plus 74 mM for the Na ion ion. Employing this Na ion selective microelectrode, the alteration of intracellular Na ion concentration following the application of ouabain in the medium will be measured. Simultaneously the contractile tension will be measured employing the RCA transducer tube. By correlating these two parameters, the relationship between an increase in intracellular Na ion concentration and that of contractility will be determined. Also some other drugs which are known to inhibit Na ion minus K ion-ATPase will be investigated. The potency of drugs in the inhibitory effect and that of inotropic effect will be compared to see whether a parallel relation holds. In addition to the above relationship, the effect of cardiac glycosides on other cellular components involved with Ca ions movement during the excitation-contraction coupling will be studied.