Human mesenchymal stem cells (hMSCs) are non-hemapoietic stem cells possessing multidifferential potential. One possible application of the hMSCs is the repair of high conductance Ca2+-sensitive voltage-dependent K+ channels (Slo1 BKCa or Maxi-K+ channels) function in vascular smooth muscle cells (SMC) damaged by oxidative stress following ionizing irradiation (Soloviev et al., 2010). Ionic channels are important for hMSCs differentiation but there is a little information on electrophysiological characteristics (Li, Deng, 2011) and function of hMSCs under oxidative stress. Experimental design of the study comprised patch-clamp technique and RT-PCR analysis. Bone marrow was aspirated in heparin from the sternum of 4 human male healthy volunteers of ages 38 – 44 years after informed consent and Institute Hematology and Transfusion Ethics Committee approval. To reduce the burning effect of lidocaine solution, local anesthesia was achieved using 1% lidocaine (up to 4 microgram/kg) in 1% sodium bicarbonate solution injected subcutaneously and into periosteum. hMSCs derived from bone marrow were separated using negative selection procedure with monoclonal antibodies and cultured as monolayer 20 – 32 days. At the end of cultivation hMSC were detached from the bottom of the flasks using trypsin-EDTA and the suspension was prepared for electrophysiological studies. Phenotypic RT-PCR analysis showed that hMSCs were negative for hematopoietic cell markers (CD14, CD34, CD45) and positive for hMSCs markers (CD29, CD44, CD71, CD73, CD90, CD105, CD166). Membrane currents were measured in the whole cell configuration of the patch clamp technique at 210 C. hMSCs were stimulated with increasing depolarizing 10 mV voltage steps 300 ms duration from a holding potential of – 60 mV. All hMSCs demonstrated a rapidly activating at potentials positive to +10 mV non-inactivated outward currents with noisy oscillation typical for Ca2+-actvated K+ currents. Mean current density in hMSCs was 41 ± 3 pA/pF at +70 mV vs. 71 ± 6 pA/pF in rat thoracic aorta SMC, and membrane potential was – 72 ± 5 mV vs. – 51 ± 3 mV in SMC. The currents were identified as carrying through BKCa channels preferentially since severely inhibited by externally applied BKCa inhibitor, paxilline (500 nM), from 41 ± 3 pA/pF to 10 ± 2 pA/pF (P<0.05) vs. 71 ± 6 and 23 ± 2 pA/pF (P<0.05) in SMC. hMSCs capacitance as a measure of cell size was 44 ± 5 pF vs. 4 ± 0.3 pF in SMC. Xanthine (X, 0.1 mM) plus xanthine oxidase (XO, 0.015 U/ml) were used to generate reactive oxygen species (ROS). 15 s of radicals exposure results in decrease of outward current amplitude from 41 ± 3 pA/pF to 13 ± 1 pA/pF (P<0.05). Whole body ionized irradiation (6 Gy) – induced oxidative stress decreased outward currents in SMC from 71 ± 6 to 24 pA/pF (P<0.05). Paxilline application was without effect in both hMSC and SMC treated with X – XO combination or irradiation, respectively, suggesting the absence or inability of BKCa channels in plasma membranes. In conclusion, ROS possess the ability to inhibit Slo1 BKCa channels embedded into undifferentiated hMSCs plasma membrane in a manner similar to damaging effect of ionized irradiation on BKCa in SMC.