The endothelial lining is a very important system playing a crucial role in vascular tone regulation and is thought to be responsible for vasospasm development. Although it is known (Soloviev et al. 2003) that radiation selectively inhibits the EDRF/NO-dependent component of acetylcholine-induced vascular relaxation, while EDHF is able to maintain endothelium-dependent relaxation at a reduced level, the underlying mechanisms of this phenomenon remain unknown. The goal of this study was to evaluate the influence of single dose, whole-body γ-irradiation (6 Gy) on large-conductance Ca²⁺-dependent potassium channels (BK_Ca channels) in primary cultured rat coronary artery endothelial cells using the whole-cell configuration of the patch-clamp technique. During irradiation, Wistar rats (250 g b.w.) were restrained in a plastic box, and the radiation beam was focused on the animal’s chest. There was no change in housing, standard food or drinking water following irradiation. The animals were closely observed for unwanted effects and there were no visible signs of discomfort or illness. On the 9th and 30th days post-irradiation the hearts were taken from the animals anesthetized with ketamine/xylazine (1 ml/kg b.w.) to obtain the coronary artery endothelial cells. The stimulation of control cells by increasingly depolarized voltage steps at potentials more positive than -40 mV elicited outward potassium currents that were followed by clearly expressed outward tail currents. The current-voltage relationships for peak and steady-state currents exhibited close to linear behaviour at potentials more negative then -40 mV and became non-linear at potentials above -40 mV. The reversal potential of the whole-cell current was -52±3 mV (n=8). A blocker of BK_Ca channels, paxillin (50 nM), abolished these currents consistent with the activation of large conductance BK_Ca channels. Irradiation significantly suppressed BK_Ca channels. Outward currents in irradiated cells isolated on the 9th day post-irradiation displayed two types – in one group of cells (20%) outward currents were suppressed significantly but not totally (type 1), while the remaining cells (80%) demonstrated total loss of outward currents (type 2). The reversal potentials for these whole-cell currents were -40±4 (n=7) and -10±2 mV (n=7), respectively. Paxillin was without effect on type 2 endothelial cells, and cells obtained on the 30th day post-irradiation, suggesting that BK_Ca channels were abolished. Irradiated cells displayed an almost linear I-V relationship indicating the absence of outwardly rectifying conductance. Increasing the extracellular potassium concentration from 5 to 10 mM decreased outward potassium currents induced by voltage ramp from -125 to +75 mV in both control and irradiated type 1 cells at potentials above -40 mV. A two-fold increase in potassium concentration led to a small increase in current at potentials lower than -75 mV, suggesting these cells expressed some inward rectifier channels. The data obtained indicate that non-fatal, whole-body γ-irradiation suppresses large conductance, calcium-activated potassium channels, which control the driving force for Ca²⁺ entry and related NO synthesis in endotheliocytes. This may contribute to endothelium disorders following radiation impact.