Surface evoked and single unit responses to vibrissa stimulation were investigated in the PosteroMedial Barrel SubField (PMBSF) area of rat and mouse SI cortex. The data strongly suggest a correspondence between the focal point of the surface projection region and the underlying barrel hollow. Units activated by the whisker movement were recorded in all cortical layers. They responded mainly in a phasic manner. Units sensitive to direction of stimulation as well as units receiving functional convergence from more than one vibrissa are described.