Glial fibrillary acidic protein (GFAP) is the main component of intermediate filaments in astrocytes. To assess its function in astrocyte swelling, we compared astrocyte membrane properties and swelling in spinal cord slices of 8- to 10-day-old wild-type control (GFAP+/+) and GFAP-knockout (GFAP−/−) mice. Membrane currents and K+ accumulation around astrocytes after a depolarizing pulse were studied using the whole-cell patch-clamp technique. In vivo cell swelling was studied in the cortex during spreading depression (SD) in 3 to 6-month-old animals. Swelling-induced changes of the extracellular space (ECS) diffusion parameters, i.e., volume fraction α and tortuosity λ, were studied by the real-time iontophoretic tetramethylammonium (TMA+) method using TMA+-selective microelectrodes. Morphological analysis using confocal microscopy and quantification of xy intensity profiles in a confocal plane revealed a lower density of processes in GFAP−/− astrocytes than in GFAP+/+ astrocytes. K+ accumulation evoked by membrane depolarization was lower in the vicinity of GFAP−/− astrocytes than GFAP+/+ astrocytes, suggesting the presence of a larger ECS around GFAP−/− astrocytes. Astrocyte swelling evoked by application of 50 mM K+ or by hypotonic solution (HS) produced a larger increase in [K+]e around GFAP+/+ astrocytes than around GFAP−/− astrocytes. No differences in α and λ in the spinal cord or cortex of GFAP+/+ and GFAP−/− mice were found; however, the application of either 50 mM K+ or HS in spinal cord, or SD in cortex, evoked a large decrease in α and an increase in λ in GFAP+/+ mice only. Slower swelling in GFAP−/− astrocytes indicates that GFAP and intermediate filaments play an important role in cell swelling during pathological states. GLIA 35:189–203, 2001. © 2001 Wiley-Liss, Inc.