The extracellular matrix molecule complex stabilized by Bral1 at the nodes of Ranvier might play a role in neuronal conduction as this complex represents a reservoir and/or diffusion barrier for Na+ ions. We measured the apparent diffusion coeffi cient of water (ADCW) by diffusion-weighted MRI and the extracellular space (ECS) diffusion parameters volume fraction α (α = ECS volume/total tissue volume) and tortuosity λ (λ2 = free/apparent diffusion coeffi cient) by the real-time iontophoretic method. Measurements were done in the somatosensory cortex and along the mediolateral (x), rostrocaudal (y) and ventrodorsal (z) axes in the corpus callosum (CC) of Bral1 +/+ and -/- mice. In the cortex, we did not fi nd any signifi cant differences in the ECS diffusion parameters or ADCW values between Bral1 +/+ and -/- mice. In the CC, λ in Bral1 +/+ mice was signifi cantly higher along all three axes (λx = 1.41 ± 0.03; λy = 1.70 ± 0.02; λz = 1.72 ± 0.01; mean ± SEM) compared to -/- mice (λx = 1.32 ± 0.02; λy = 1.56 ± 0.01; λz = 1.58 ± 0.02); there were no signifi cant differences in α. These data were confi rmed by a lower ADCW in the CC of Bral1 +/+ than of -/- mice, along the x (1 158 ± 55 μm2 s-1 and 1 340 ± 25 μm2 s-1, respectively) and y axes (442 ± 19 μm2 s-1 and 521 ± 24 μm2 s-1, respectively). We conclude that the deletion of Bral1 results in a reduction of diffusion barriers at the nodes of Ranvier, which in turn facilitates diffusion in all directions in the myelinated white matter.