The [Ca2+]-activated photoprotein aequorin was used to measure [Ca2+] in canine cardiac Purkinje fibers during the positive inotropic and toxic effects of ouabain, strophanthidin, and acetylstrophanthidin. The positive inotropic effect of these substances was associated with increases in the two components of the aequorin signal, L1 and L2. On the average, strophanthidin at 10(-7) M produced steady, reversible increases in L1, L2, and peak twitch tension of 20, 91, and 240%, respectively. This corresponds to increases in the upper-limit spatial average [Ca2+] from 1.9 X 10(-6) M to 2.1 X 10(-6) M at L1 and from 1.4 X 10(-6) M to 1.8 X 10(-6) M at L2. Elevation of diastolic luminescence above the control level was not detected. At higher concentrations (5 X 10(-7) M), strophanthidin produced aftercontractions, diastolic depolarization, and transient depolarizations, all of which were associated with temporally similar changes in [Ca2+]. During these events, diastolic [Ca2+] rose from the normal level of approximately 3 X 10(-7) M up to 1-2 X 10(-6) M. The negative inotropic effect of 5 X 10(-7) M strophanthidin was not associated with a corresponding decrease in the [Ca2+] transient but was associated with a change in the relationship between [Ca2+] and tension. Assuming the Na+-lag mechanism of cardiotonic steroid action, we conclude the following: at low concentrations of drug, increased Ca2+ uptake by the sarcoplasmic reticulum prevents a detectable rise in cytoplasmic [Ca2+] during diastole, but this increased Ca2+ uptake results in increased release of Ca2+ during the action potential. At higher drug concentrations, observable [Ca2+] changes during diastole activate tension and membrane conductance changes.