A rare mutation affecting the Forkhead-box protein P2 (FOXP2) transcription factor causes a severe monogenic speech and language disorder. Mice carrying an identical point mutation to that observed in affected patients (Foxp2^+/R552Hmice) display motor deficits and impaired synaptic plasticity in the striatum. However, the consequences of the mutation on neuronal function, in particular in the cerebral cortex, remain little studied. Foxp2 is expressed in a subset of Layer VI cortical neurons. Here, we used Ntsr1-EGFP mice to identify Foxp2+ neurons in the mouse auditory cortexex vivo.We studied the functional impact of the R552H mutation on the morphologic and functional properties of Layer VI cortical neurons from Ntsr1-EGFP; Foxp2^+/R552Hmale and female mice. The complexity of apical, but not basal dendrites was significantly lower in Foxp2^+/R552Hcortico-thalamic neurons than in control Foxp2^+/+neurons. Excitatory synaptic inputs, but not inhibitory synaptic inputs, were decreased in Foxp2^+/R552Hmice. In response, homeostatic mechanisms would be expected to increase neuronal gain, i.e., the conversion of a synaptic input into a firing output. However, the intrinsic excitability of Foxp2+ cortical neurons was lower in Foxp2^+/R552Hneurons. A-type and delayed-rectifier (DR) potassium currents, two putative transcriptional targets of Foxp2, were not affected by the mutation. In contrast, GABA_B/GIRK signaling, another presumed target of Foxp2, was increased in mutant neurons. Blocking GIRK channels strongly attenuated the difference in intrinsic excitability between wild-type (WT) and Foxp2^+/R552Hneurons. Our results reveal a novel role for Foxp2 in the control of neuronal input/output homeostasis.SIGNIFICANCE STATEMENTMutations of the Forkhead-box protein 2 (FOXP2) gene in humans are the first known monogenic cause of a speech and language disorder. The Foxp2 mutation may directly affect neuronal development and function in neocortex, where Foxp2 is expressed. Brain imaging studies in patients with a heterozygous mutation in FOXP2 showed abnormalities in cortical language-related regions relative to the unaffected members of the same family. However, the role of Foxp2 in neocortical neurons is poorly understood. Using mice with a Foxp2 mutation equivalent to that found in patients, we studied functional modifications in auditory cortex neuronsex vivo. We found that mutant neurons exhibit alterations of synaptic input and GABAB/GIRK signaling, reflecting a loss of neuronal homeostasis.