Carbenoxolone (CBX), a derivative of glycyrrhizic acid, is used in the treatment of gastritis and ulcer and is also known as a potent blocker of gap junctions (GJ) that play an important role in synchronized neural activity. In this study we explored the effects of CBX on field excitatory postsynaptic potentials (fEPSPs) and long-term potentiation (LTP) of fEPSPs in the hippocampal Schaffer collateral - CA1 pathway as the two forms of neuronal cooperative activity. Bath application of 100 mM CBX to mouse hippocampal slices resulted in long-lasting depression of field EPSPs, while the lower concentrations caused reversible depression (50 mM) or no effect (10 mM). CBX at 50 mM applied 15 min before tetanus prevented the induction of LTP by a single train of high-frequency stimulation and significantly reduced LTP elicited by theta-burst stimulation. Long preincubation with 10 mM CBX caused similar impairment of electrically-induced LTP without significant changes in basic synaptic properties. The treatment with CBX did not affect, however, the long-term enhancement of fEPSPs induced by 30-min perfusion with 10 mM taurine. The data suggests the involvement of GJs in the generation of hippocampal field potentials and tetanus-induced LTP. Supported by DFG (SFB575) and RFBR (04-04-49022) grants.