Simultaneous assessment of synaptic activity and glutamate efflux in guinea pig hippocampal brain slices was made before, during and after a 10-min period of hypoxia. Spontaneous glutamate efflux was assessed by determining glutamate concentration in the superfusion medium at discrete times using high performance liquid chromatography (HPLC). Synaptic activity was assessed using extracellular recording of the evoked population spike in the CA1 region following stimulation of the Schaffer collateral pathway. Hypoxia decreased (P < 0.05) the amplitude of the population spike by 3 min and abolished it by 5 min. This was accompanied by an increase (P < 0.05) in glutamate concentration in the superfusate at 10 min. Following re oxygenation, the population spike returned to baseline amplitude by 5 min and was greater (P < 0.05) than baseline at 10 and 20 min of recovery. Glutamate concentration returned to baseline levels by 1 min of recovery. This experimental preparation can be used to explore the temporal relationship between glutamate efflux and synaptic activity during hypoxia. The results of this study indicate that, in the hippocampal CA1 region, post-synaptic elements are more sensitive than their presynaptic counterparts to hypoxia.