Astrocytes, the major type of non-neuronal cells in the brain, play an important functional role in extracellular potassium ([K+]o) and pH homeostasis. Pathological brain states that result in [K+]o and pH dysregulation have been shown to cause astrocyte swelling. However, whether astrocyte volume changes occur under physiological conditions is not known. In this study we used two-photon imaging to visualize real-time astrocyte volume changes in the stratum radiatum of the hippocampus CA1 region. Astrocytes were observed to swell by 19.0±0.9% in response to a small physiological increase in the concentration of [K+]o (3 mM). Astrocyte swelling was mediated by the influx of bicarbonate (HCO3−) ions as swelling was significantly decreased when the influx of HCO3− was reduced. We found: 1) in HCO3− free extracellular solution astrocytes swelled by 5.4±0.7%, 2) when the activity of the sodium-bicarbonate cotransporter (NBC) was blocked the astrocytes swelled by 8.3±0.7%, and 3) in the presence of an extracellular carbonic anhydrase (CA) inhibitor astrocytes swelled by 11.4±0.6%. Because a significant HCO3− efflux is known to occur through the γ-amino-butyric acid (GABA) channel, we performed a series of experiments to determine if astrocytes were capable of HCO3− mediated volume shrinkage with GABA channel activation. Astrocytes were found to shrink −7.7±0.5% of control in response to the GABAA channel agonist muscimol. Astrocyte shrinkage from GABAA channel activation was significantly decreased to −5.0±0.6% of control in the presence of the membrane-permeant CA inhibitor acetazolamide (ACTZ). These dynamic astrocyte volume changes may represent a previously unappreciated yet fundamental mechanism by which astrocytes regulate physiological brain functioning.