>The purine nucleoside adenosine is released during seizure activity and has been described as an endogenous anticonvulsant that raises the threshold for seizure generation or aids in the termination of seizures. The proposed neuroprotective properties of adenosine are primarily achieved through inhibition of glutamate release via presynaptic A₁ adenosine receptors. In an in vitro model of epilepsy, using 600 µm transverse hippocampal slices prepared from 16-22 day old rats killed humanely by cervical dislocation, seizures were induced by means of a 2 s, 60 Hz stimulus train delivered to stratum radiatum of the CA1 region in nominally Mg²⁺-free aCSF at 30−31 °C. In control conditions the stimulus initiated epileptiform activity, which persisted for 21.3 ± 3.4 s (mean ± SEM; n=10) and caused a profound (79.7 ± 3.8%; n=10), transient (5−6 min) depression of synaptic transmission. Upon application of CPT, a selective adenosine A₁ receptor antagonist, the duration of evoked seizures was dramatically prolonged (41.0 ± 7.3s; paired t-test; p = 0.002) whilst the depression of synaptic transmission was significantly reduced (28.0 ± 7.3%; paired t-test; p < 0.001). This confirms that adenosine is released and inhibits seizures via activation of A₁ receptors. To measure directly the release of adenosine during epileptiform activity we used a miniature adenosine sensor, which relies on the enzymatic conversion of adenosine to HO₂ (Dale, 1998; Dale et al., 2000). The net signal obtained from the sensor, placed on the surface of area CA1, revealed that extracellular adenosine rises rapidly following the stimulus train. Within one minute of the onset of the seizure the peak level of adenosine measured 2.4 ± 0.6 µM (n = 4) and the time course of recovery from the post-ictal depression of synaptic transmission corresponded closely with the clearance of endogenous adenosine. Adenosine release was also observed during spontaneous epileptiform activity, either in control conditions or in the presence of CPT, and argues against stimulus-induced electroporation of axons as the source of extracellular adenosine. These results reveal the time course of adenosine release during brief periods of epileptiform activity and strengthen the importance of endogenous adenosine in the termination of seizures.