Combined actions of GM1 ganglioside molecules and Ca2+ ions on long-term potentiation (LTP) of the hippocampal CA1 synapses were examined in slice preparations. Application of exogenous GM1 ganglioside significantly enhanced LTP. Antigenic blocking of endogenous GM1 ganglioside by applying anti-GM1 antiserum appeared to attenuate LTP. Exogenous GM1 treatment not only antagonized LTP suppression caused by low Ca2+ (1.0 microM) in the perfusion medium, but also amplified LTP at high Ca2+ (5.0 microM) condition. Thus, GM1 may stabilize intracellular calcium levels to enhance synaptic potentiation.