To understand the neuropathological roles of free radicals we investigate their actions in a model neuronal system, the hippocampal brain slice. Free radicals can be generated through a number of methods: hydrogen peroxide to produce hydroxyl radicals, dihydroxyfumarate to generate superoxide and ionizing radiation producing a variety of radical species. We find that free radicals have a number of profound effects in this system, which can be prevented by free-radical scavengers and antioxidants. With exposure to free radicals, the ability to generate spikes and synaptic efficacy are impaired. Decreased spike generating ability is correlated with lipid peroxidation. No change in membrane potential, membrane resistance, or many of the potassium currents can account for the effect on spike generation. Protein oxidation is likely to underlie synaptic damage. Both inhibitory and excitatory synaptic potentials are reduced by free-radical exposure. Presynaptic mechanisms are implicated. Lower concentrations of radicals prevent the maintenance of long-term potentiation, perhaps through oxidation of the NMDA receptor. The actions of the free radicals are often reversible because of the presence of repair mechanisms, such as glutathione, in hippocampal slices. The brain slice preparation has allowed us to begin to understand the electrophysiological and biochemical consequences of free-radical exposure.