Tonic inhibition mediated by extrasynaptic GABAA receptors is a powerful conductance that controls cell excitability. Throughout the CNS, tonic inhibition is expressed at varying degrees across different cell types. Despite a rich history of cortical interneuron diversity, little is known about tonic inhibition in the different classes of cells in the cerebral cortex. We therefore examined the cell-type specificity and functional significance of tonic inhibition in layer 4 of the mouse somatosensory barrel cortex. In situ hybridization and immunocytochemistry showed moderate &#948;-subunit expression across the barrel structures. Whole-cell patch-clamp recordings additionally indicated that significant levels of tonic inhibition can be found across cell types, with differences in the magnitude of inhibition between cell types. To activate tonic currents, we used 4,5,6,7-tetrahydroisothiazol-[5,4-c]pyridin-3-ol (THIP, a super-agonist at -subunit containing GABA_A receptors) at a concentration which did not affect synaptic decay kinetics. THIP produced greater shifts in baseline holding current in inhibitory cells (low threshold spiking (LTS), 109±17pA; fast spiking (FS), 111±15pA) than in excitatory cells (39±10pA; p<0.001). In addition to these differences across cell-types, there was also variability within inhibitory cells. FS cells with faster action potentials had larger baseline shifts. As FS cells are known mediators of feedforward inhibition, we tested whether THIP induced tonic conductance selectively controls feedforward circuits. THIP application resulted in the abolishment of the IPSP in thalamic-evoked disynaptic responses in a subset of excitatory neurons. These data suggest multiple feedforward circuits can be differentiated by the inhibitory control of the presynaptic inhibitory neuron.