Primary somatosensory potentials were evoked in the cerebral cortex of awake rats by stimulation of the superficial radial nerve. Morphine injected intravenously in a dose of 2 mg kg produced enhancement of the positive and the negative wave of the evoked potential, the effect on the negative wave being much more pronounced than the effect on the positive wave. The maximal effect of morphine developed more rapidly when halothane anesthesia was employed for the surgical procedure instead of pentobarbital anesthesia. Direct cortical responses elicited by stimulation of the cortical surface were also enhanced by morphine. The time course and the extent of the effect of morphine on the direct cortical and on the primary evoked response, however, did not run parallel. Levallorphan injected intravenously in a dose of 0.2 mg kg inhibited considerably more the effect of morphine on the evoked potential than on the direct cortical response. It is suggested that morphine enhances the primary somatosensory evoked potential by increasing the excitability of cortical neurones, an effect in which facilitatory processes in the thalamocortical system may be involved.