We investigated the force-frequency relationship (0.5-3 Hz) in non-failing human myocardium and in end-stage failing human myocardium due to dilated cardiomyopathy or subacute myocarditis. In non-failing myocardium, force of contraction increased with increasing stimulation frequency. In end-stage heart failure, the force-frequency relationship was inverse in myocardium from dilated cardiomyopathy, but was similar to control in myocardium from subacute myocarditis. After increasing extracellular Ca(2+)-concentration from 2.5 to 7.2 mM, the shape of the force-frequency relationship was not changed in nonfailing myocardium. In dilated cardiomyopathy, the decline in force with increasing frequencies was even more pronounced at 7.2 mM compared to 2.5 mM extracellular Ca2+. In subacute myocarditis, at Ca2+ 7.2 mM, increasing frequencies increased force in the lower frequency range (less than 1.75 Hz) only, whereas at higher stimulation rates force declined again. These results indicate that (1.) alterations of the force-frequency relationship in the failing human heart depend on the underlying cardiac disease and/or the time-course of the disease, and (2.) an increase in the extracellular Ca(2+)-concentration aggravates changes in the force-frequency relationship in the failing myocardium.