Our classical understanding of neurons emphasizes the role of dendrites for synaptic signal integration and plasticity. In contrast, the axon has been recognized as a rather static output device. This view is rapidly changing, with recent research revealing a much more active role for axonal microdomains in neuronal signal processing. A key regulator in this regard is the axon initial segment (AIS), strategically positioned at the proximal axon. Its molecular architecture, particularly a high density of voltage-gated ion channels, is the anatomical substrate for action potential (AP) initiation. Recent studies have identified the AIS as a significant contributor to the modulation of neuronal excitability. In fact, the AIS exhibits striking structural and functional plasticity, depending on network activity. By regulating AIS length and position, neurons can modulate their excitability and therefore contribute to maintain the functional stability of neuronal circuits. However, the in vitro and in vivo models used often follow rather drastic, non-physiological strategies. Yet if the AIS is to represent another cellular microdomain involved in fine-tuning neuronal activity, a central and presently unanswered question is whether AIS plasticity is actually taking place within a normal physiological range of neuronal function, and which time-frames are required to trigger its remodelling. So we asked whether sudden changes in network state can elicit rapid AIS plasticity (time-scale within hours) for neurons to adapt to global changes. We studied AIS plasticity in the rodent whisker-to-barrel system, specifically investigating AIS-related changes in cortical pyramidal cells of layers II/III and V in mouse primary somatosensory cortex, barrel field (S1BF). Adult mice (P28 and older, n=6 for each group) were subjected to unilateral whisker-trimming and exposed to enriched environment (EE) conditions for 1, 3 and 6 hours, triggering increased stimulation of the remaining whisker-to-barrel pathway. AIS modulations and cellular responses were analysed using multichannel immunofluorescence (against AIS scaffolding proteins ankyrinG and βIV-spectrin, sodium channels, and the immediate-early gene c-fos; n=6 per group), confocal microscopy, and whole-cell patch-clamp recordings in acute slices. Increased stimulation of the barrel network was indicated by upregulation of c-fos in layer II/III pyramidal neurons within 3 hours of EE exposure. The same neurons further showed a significant AIS length reduction in the over-stimulated S1BF compared to controls (mean and S.D. 20.52 µm ± 1.98 µm EE vs. 23.54 µm ± 1.96 µm control; n=600 AIS from 6 animals, one-way ANOVA, p<0.05). AIS length then normalized to control level 6 hours after EE (mean and S.D. 23.82 µm ± 1.96 µm EE vs. 23.31 µm ± 1.56 µm control; n=600 AIS from 6 animals, one-way ANOVA, p<0.05). Cellular excitability also changed significantly in this 3 hour time-frame: EE-exposed S1BF neurons had a significantly elevated AP threshold (mean and S.D. 480.4 ± 36.79 pA EE vs. 375.5 ± 70.5 pA control; n=9, t-test, p<0.05) and showed reduced excitability to appropriate stimuli (mean and S.D. 30.00 ± 1 Hz EE vs. 25.33 ± 1.03 Hz control; n=9, t-test, p<0.05). Our data indicates that AIS undergo rapid structural plasticity, correlated with functional consequences rendering overstimulated cells less excitable, a mechanism that seems reversible. We therefore propose that rapid AIS plasticity could serve as a fast modulator of neuronal activity in excited networks until a homeostatic balance is achieved.