Interleukin-1 (IL-1), a cytokine with a broad spectrum of biological activity, modulates electrical properties of central neurons in the brain. The effects of IL-1 beta (143 pM) on conductances opened by synaptic stimulation of the Schaffer collaterals were studied by intracellular recording of hippocampal pyramidal cells of the CA1 region. IL-1 beta enhanced and prolonged synaptic inhibition by about 2 to 3-fold. Heat-inactivated IL-1 beta had no effect. This finding implies that IL-1 beta changes interneuronal communication in the hippocampus with a possible impact on neuronal plasticity.