The role of adenosine in suppressing synaptic responses during prolonged moderate hypoxia was examined in rat hippocampal slices. The intrahypoxic loss of evoked synaptic responses could be reversed partially by an antagonist of the A1 type adenosine receptor during an entire hour of hypoxia. These findings indicate that the capacity to express synaptic transmission exists during prolonged moderate hypoxia, and that endogenous adenosine actively suppresses transmission via an action at A1 type receptors.