Because of its exceptionally robust nature, the ex vivo frog sciatic nerve has been the subject of numerous electrophysiology and MRI studies over the years. Here we report on diffusion MRI signal changes resulting from 50 and 100 Hz in-magnet electrical stimulation of perfused bullfrog sciatic nerves. The inexpensive perfusion system we have implemented allows for good long-term in-magnet stability and simultaneous MRI/electrophysiology studies. Decreases in water diffusivity and compound action potential conduction velocities accompany prolonged periods of repetitive electrical stimulation. Both of these changes are consistent with hypothesized microstructural alterations of the PNS myelin.