We investigated the role of aquaporin-4 (AQP4), a water channel expressed in glial cells, in neural activity mediated morphological changes observed in brain slice preparation. Changes in flavoprotein fluorescence (FF) and infrared light scattering (LS) signals were measured before and after repetitive stimulation of layer VI in rostral somatosensory cortical slices taken from AQP4 knockout (KO) and wild-type (WT) mice. Changes in FF, which reflect neural aerobic activities, were comparable for the two groups in all cortical layers. However, changes in LS signals, which are indicative of cell swelling, were significantly decreased in layer I of AQP4 KO mice compared to that of WT mice. We conclude that AQP4 likely plays a significant role in neural activity-dependent glial swelling.