The role of sodium and calcium ions in strophanthidin inotropy was studied by measuring simultaneously the electrical, mechanical, and intracellular sodium ion activities in electrically driven cardiac Purkinje fibers under conditions that change the intracellular sodium or calcium level (tetrodotoxin, strophanthidin, high calcium, and norepinephrine). Tetrodotoxin (TTX; 1-5 X 10(-6)M) shifted the action potential plateau to more negative values, shortened the action potential duration, and decreased the contractile tension and the intracellular sodium ion activity (aiNa). The changes in tension and in aiNa caused by TTX appear to be related since they had similar time courses. Strophanthidin (2-5 X 10(-7)M) increased tension and aiNa less in the presence of TTX, and, for any given value of aiNa, tension was less than in the absence of TTX. Increasing extracellular calcium (from 1.8 to 3.3-3.6 mM) or adding norepinephrine (0.5-1 X 10(-6)M) increased tension and decreased aiNa less in the presence than in the absence of TTX. When two of the above procedures were combined, the results were different. Thus, during the increase in aiNa and tension caused by strophanthidin in the presence of TTX, increasing calcium or adding norepinephrine increased tension markedly but did not increase aiNa further. In a TTX-high calcium or TTX-norepinephrine solution, adding strophanthidin increased both tension and aiNa, and the increase in tension was far greater than in the presence of TTX alone. The results indicate that: (a) the contractile force in Purkinje fibers is affected by a change in aiNa; (b) a decrease in aiNa by TTX markedly reduces the inotropic effect of strophanthidin, possibly as a consequence of depletion of intracellular calcium; (c) increasing calcium influx with norepinephrine or high calcium in the TTX-strophanthidin solution produces a potentiation of tension development, even if aiNa does not increase further; and (d) when the calcium influx is already increased by high calcium or norepinephrine, strophanthidin has its usual inotropic effect even in the presence of TTX. In conclusion, the positive inotropic effect of strophanthidin requires that an increase in aiNa be associated with suitable calcium availability.