Levosimendan is an inodilatory drug that mediates its cardiac effect by the calcium sensitization of contractile proteins. The target protein of levosimendan is cardiac troponin C (cTnC). In the current work, we have studied the interaction of levosimendan with Ca2+-saturated cTnC by heteronuclear NMR and small angle x-ray scattering. A specific interaction between levosimendan and the Ca2+-loaded regulatory domain of recombinant cTnCC35S was observed. The changes in the NMR spectra of the N-domain of full-length cTnCC35S, due to the binding of levosimendan to the primary site, were indicative of a slow conformational exchange. In contrast, no binding of levosimendan to the regulatory domain of cTnCA-Cys, where all the cysteine residues are mutated to serine, was detected. Moreover, it was shown that levosimendan was in fast exchange on the NMR time scale with a secondary binding site in the C-domain of both cTnCC35S and cTnCA-Cys. The small angle x-ray scattering experiments confirm the binding of levosimendan to Ca2+-saturated cTnC but show no domain-domain closure. The experiments were run in the absence of the reducing agent dithiothreitol and the preservative sodium azide (NaN3), since we found that levosimendan reacts with these chemicals, commonly used for preparation of NMR protein samples.