The blood-brain barrier (BBB) is a highly specific, lipophilic barrier between the systemic blood circulation and the central nervous system. The BBB is formed by the specialized endothelium of the brain capillaries which have an extremely low rate of transcytosis and form a restrictive barrier for paracellular diffusion due to the absence of fenestrations. Instead, brain capillary endothelial cells are interconnected by a complicated structure of tight junctions, rendering these vessels little permeable. Furthermore, it is well known that an increase of the intracellular calcium concentration ([Ca²⁺]_i) takes a central stage in an increased permeability of the BBB. Our hypothesis is that Ca²⁺ signals can be communicated between neighbouring BBB endothelial cells and in this way be decisive between a physiological and a generalized, pathological opening of the BBB. The goal of the present study was to investigate whether a low [Ca²⁺]_e is able to trigger Ca²⁺ waves and oscillations in RBE4, a cell line derived from BBB endothelial cells. This triggering effect of low [Ca²⁺]_e was already observed in astrocytes. Furthermore, we examined the mechanisms involved in this triggering of dynamic Ca²⁺ changes. More specific, since low [Ca²⁺]_e is also a known trigger for hemichannel opening, we investigated the role of hemichannels in the induction of Ca²⁺ waves and oscillations. These channels consist of six connexin (Cx) proteins and create a direct link between the cytoplasm and the extracellular environment, providing a possible release pathway for Ca²⁺ mobilizing messengers involved in paracrine Ca²⁺ communication. We studied Ca²⁺ dynamics in the presence of buffers with various low [Ca²⁺], ranging from 0.1 nM to 200 mM free Ca²⁺, by means of the cell permeable, fluorescent, Ca²⁺ sensitive dye Fluo3-AM. The presence of hemichannels was investigated via ATP release and dye uptake. Our data show that all low Ca²⁺ buffers trigger Ca²⁺ waves and oscillations in RBE4 cells over a time period of 10 min. This triggering effect was within 1 minute reduced by the Cx43 mimetic peptide Gap27. Expression of this Cx subtype was demonstrated in RBE4. Our results also show that low [Ca²⁺]_e triggers ATP Release and dye uptake in RBE4 and this was again inhibited by Gap27, suggesting the presence of Cx43 hemichannels. All taken together, our data suggest a possible involvement of Cx43 hemichannels in the induction of Ca²⁺ waves and oscillations in endothelial cells.