Adenosine is a potent modulator of neuronal activity throughout the nervous system. Previous evidence suggests that adenosine presynaptically inhibits synaptic transmission. The mechanism of presynaptic inhibition is uncertain. Adenosine may inhibit transmitter release by reducing voltage-dependent Ca2+ currents, activating K+ currents, or by mechanisms downstream to Ca2+ influx. By simultaneously recording the presynaptic Ca2+ transient ([Ca]t) and the field excitatory postsynaptic potential (fEPSP) at CA3-CA1 synapses of hippocampal slices, we found that adenosine, through activation of presynaptic A1 receptors, inhibits the fEPSP primarily by reducing the [Ca]t. Reduced [Ca]t was due to inhibition of omega-conotoxin GVIA-sensitive and some unidentified Ca2+ channels, probably including Q-type, but not to omega-agatoxin-IVA-sensitive Ca2+ channels. The adenosine A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine enhanced both the [Ca]t and the fEPSP, suggesting a tonic inhibition of the [Ca]t by endogenous adenosine in the hippocampal slice.