Levosimendan is approved for use as an intravenous agent to treat temporarily patients with severe systolic heart failure. It is thought, but hitherto never tested in isolated human cardiac tissue, that the main metabolite of levosimendan, called OR 1896, might cause the positive inotropic effect of levosimendan in patients. For comparison, we studied mouse atrial preparations. OR alone (up to 10 µM) failed to increase force of contraction in mouse atrial preparations while in the presence of the phosphodiesterase (PDE) IV-inhibitor rolipram (0.1 µM) profound positive inotropic effects and positive chronotropic effects of OR 1895 were recorded in mouse atrial preparations. In contrast to mouse atrium, we noticed that OR 1896 alone (0.01 µM to 10 µM cumulatively applied) exerted time- and concentration-dependent positive inotropic effects 1 µM increase by 72 ± 14.7%, p<0.05, n=7) and shortened the time of relaxation (by 7 .3 %, p<0.05, n=7) in isolated electrically paced human right atrial muscle preparations (obtained during cardiac surgery), that started at 0.1 µM and plateaued at 1 µM OR 1896. The positive inotropic effect of OR 1896 was antagonized by 10 µM propranolol. The positive inotropic effect of OR 1896 in mouse atrial preparations and human right atrial preparations was less effective than that of 1 µM isoprenaline (human atrium by 154 %). The positive inotropic effect of OR 1896 was diminished by pre-incubation of the samples with the PDE III inhibitor cilostamide (10 µM). EMD 57033 (10 µM), a calcium sensitizer, increased force of contraction further in the additional presence of 1 µM OR 1896. Our data suggest that OR 1896 is indeed a positive inotropic agent in the human heart and that OR 1896 acts mainly as a PDE III – inhibitor because the effect of OR 1896 was attenuated by a β-adrenoceptor antagonist and thus OR 1896 is unlikely to act as a calcium sensitizer in the human heart.