A novel form of spreading acidification and depression in the rat cerebellar cortex was imaged in vivo using the pH-sensitive dye, Neutral red. Surface stimulation evoked an initial beam of increased fluorescence (i.e., decreased pH) that spread rostrally and caudally across the folium and into neighboring folia. A transient but marked suppression in the excitability of the parallel fiber-Purkinje cell circuitry accompanied the spread. Characteristics differentiating this phenomenon from the spreading depression of Leao include: high speed of propagation on the surface (average of 450 microm/s), stable extracellular DC potential, no change in blood vessel diameter, and repeatability at short intervals. This propagating acidification constitutes a previously unknown class of neuronal processing in the cerebellar cortex.