THE function of the projection from the secondary somatosensory cortex (SII) to the primary somatosensory cortex (SI) in rats was investigated by recording sensory evoked potentials (SEP) in SI during glutamate activation and lidocaine blockade of SII. In anesthetized animals, glutamate stimulation of SII decreased SEP latency and increased SEP amplitude, whereas no changes were evident during lidocaine blockade of SII. In awake animals, a second, later component of the SEP appeared. This second component was almost completely eliminated during lidocaine blockade of SII. We conclude that the projection from SII to SI in rats slightly facilitates the SEP response in anesthetized animals and is responsible for a major portion of the late component of the SEP in awake animals.