Human mesenchymal stem cells (hMSC) have gained considerable interest due to their potential use for cell replacement therapy and tissue engineering. One strategy is to differentiate these bone marrow stem cells in vitro into cardiomyocytes prior to implantation. In this context ion channels can be important functional markers of cardiac differentiation. At present there is little information about the electrophysiological behaviour of the undifferentiated hMSC. We therefore investigated mRNA expression of 26 ion channel subunits using semiquantitative RT-PCR and recorded transmembrane ion currents with the whole-cell voltage clamp technique. Bone marrow hMSC were obtained from healthy donors. The cells revealed a distinct pattern of ion channel mRNA with high expression levels for some channel subunits (e.g. Kv4.2, Kv4.3, MaxiK, HCN2, and α1C of the L-type calcium channel). Outward currents were recorded in almost all cells. The most abundant outward current rapidly activated at potentials positive to +20 mV. This current was identified as a large-conductance voltage- and Ca2+-activated K+ current, conducted by MaxiK channels, due to its high sensitivity to tetraethylammonium (IC50= 340 μm) and its inhibition by 100 nm iberiotoxin. A large fraction of cells also demonstrated a more slowly activating current at potentials positive to –30 mV. This current was selectively inhibited by clofilium (IC50= 0.8 μm). Ba2+ inward currents, stimulated by 1 μm BayK 8644 were found in a few cells, indicating the expression of functional L-type Ca2+ channels. Other inward currents such as sodium currents or inward rectifier currents were absent. We conclude that undifferentiated hMSC express a distinct pattern of ion channel mRNA and functional ion channels that might contribute to physiological cell function.