1. The spatial patterns of activation in the rat cerebellar cortex evoked by peripheral stimulation were studied in vivo using optical imaging techniques. 2. Crus I and Crus II were stained with the pH sensitive dye, neutral red. Electrical stimulation of the vibrissae area of the ipsilateral face evoked optical responses consisting of parasagittal bands. The bands were 100-300 μm in width, elongated in the anterior-posterior direction, commonly extended across at least two folia, and varied in number from 1 to 7. 3. The optical responses were dependent on activation of postsynaptic elements since they were decreased substantially by the non-N-methyl-D- aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione. The optical bands were shown to correspond anatomically with the parasagittal compartments revealed by immunostaining with anti-zebrin II. 4. The present study demonstrates that functional parasagittal compartments exist in the rat cerebellar cortex and suggests that zebrin-positive Purkinje cell subgroups are anatomically related to this functional organization.