In the in-vivo brain background synaptic activity has a strong modulatory influence on neuronal excitability. Here we report that in rat hippocampal slices, blockade of endogenous in vitro background activity results in an increased excitability of CA1 pyramidal neurons within tens of minutes. The increase in excitability constitutes a leftward shift in the input-output relationship of pyramidal neurons, indicating a reduced threshold for the induction of action potentials. The increase in excitability results from an adaptive decrease in a sustained K⁺ conductance, as recorded from somatic cell-attached patches. After 20 minutes of blockade of background activity, the mean sustained K⁺ current amplitude in somatic patches was reduced to 46 ± 9% of that in time-matched control patches. Blockade of background activity did not affect fast Na⁺ conductance. Together, these results suggest that the reduction in K⁺ conductance serves as an adaptive mechanism to increase the excitability of CA1 pyramidal neurons in response to changes in background activity such that the dynamic range of the input-output relationship is effectively maintained.