Levosimendan is hypothesized to be primarily a calcium sensitizer in vitro. Therefore, its inotropic action may be similar in both the normal and the congestive heart failure (CHF) state, and it may be associated with a decreased energetic cost of inotropism in vivo. To test these hypotheses, we gave levosimendan to cross-circulated isolated hearts from normal (n = 11) and CHF (n = 7, 4-week rapid pacing) dogs. Peak isovolumic left ventricular pressure at an end-diastolic pressure of 5 mm Hg (Pmax,5) measured by an intraventricular balloon was 120 +/- 15 mm Hg in normal dogs, and it was increased by approximately 40% in response to approximately 0.63 microM levosimendan. In CHF dogs, base-line Pmax,5 was only 60 +/- 12 mm Hg (P < .01 compared to normals), and approximately 8.4 microM levosimendan (P < .05) was required to increase Pmax,5 by approximately 40%. The inotropic actions were associated with increases in unloaded myocardial oxygen consumption by comparable amounts in normal and falling hearts. The blunted inotropic response in CHF and the energetic cost of inotropism were also comparable to those obtained with isoproterenol. In other studies, there was no significant inotropic action of levosimendan in Langendorff-perfused rat hearts (n = 5), and intracellular calcium concentration, estimated by macroinjected aequorin, in ferret hearts (n = 2) increased dose-dependently. These findings suggest that inotropic actions of levosimendan in vivo may be mediated in part by factors other than calcium sensitization.