Hippocampal pyramidal cell excitability is regulated both by fast synaptic inhibition and by tonically active high-affinity extrasynaptic GABA_Areceptors. The impact of tonic inhibition on neuronal gain and offset, and thus on information processing, is unclear. Offset is altered by shunting inhibition, and the gain of a neuronal response to an excitatory input can be modified by changing the level of “background” synaptic noise. Therefore, tonic activation of GABA_Areceptors would be expected to modulate offset and, in addition, to alter gain through a shunting effect on synaptic noise. Here we show that tonically active GABA_Areceptors in CA1 pyramidal cells show marked outward rectification, while the peaks of IPSCs exhibit a linear current–voltage relationship. As a result, tonic GABA_Areceptor-mediated currents have a minimal effect upon subthreshold membrane potential variation due to synaptic noise, but predominantly affect neurons at spiking threshold. Consistent with this, tonic GABA_Areceptor-mediated currents in pyramidal cells exclusively affect offset and not gain. Modulation of tonically active GABA_Areceptors by fluctuations in extracellular GABA concentrations or neuromodulators acting on high-affinity receptors potentially provides a powerful mechanism to alter neuronal offset independently of neuronal gain.