Schaffer collaterals of rat and mouse hippocampal slices were stimulated with bursts of pulses (300 Hz for 50 ms, 2-s intervals) for 30-s which caused a stable increase in the size of the population spike known as long-term potentiation. The release of adenosine triphosphate (ATP) was measured with a luciferase-luciferine system and the light emitted was recorded with a photomultiplier placed beneath a modified slice chamber. ATP release was observed shortly after the start of stimulation and was quantified by comparison with the response of standard solutions of ATP. No ATP release was observed in a Ca2+ free solution or after low frequency stimulation (1 Hz). Glutamate (2 mM), applied without electrical stimulation, did not evoke ATP release. Also, the glutamate receptor blocker, kynurenic acid (10 mM), did not block ATP release. It is concluded that ATP is released from electrically stimulated hippocampal slices from presynaptic nerve terminals in a calcium-dependent fashion and may play a role in the modulation of synaptic efficiency.