Cortical and thalamic bioelectrical activity was studied following topical application of 1% morphine solution to the primary somatosensory area (PSA) and the parietal association area (PAA) of the rat cerebral cortex. At the cortical level, morphine enhanced both the spontaneous and the somatosensory evoked activity of PSA and PAA. The changes observed were restricted to the cortical region where the drug was applied. At the thalamic level, administration of morphine to the PSA or PAA significantly increased the amplitude of the evoked responses recorded from the center median-parafascicular complex (CM-Pf). It is concluded that morphine has a direct excitatory influence on cortical neurons involved in somatosensory processes. This morphine-induced cortical activation generates descending influences modulating neuronal activity at the thalamic CM-Pf complex level.