Treatment with varying doses of morphine and its antagonist naloxone produced different response patterns in "spontaneous" multiunit discharges recorded from the medial thalamus and caudate nucleus of freely behaving rats previously implanted, stereotaxically, with permanent semimicro-electrodes. The changes in electrical discharges induced by incremental doses of morphine exhibited dose-related patterns, and could be reversed by naloxone. This procedure, testing several incremental doses of a drug, provides a tool with which to identify and classify the specific response patterns induced by morphine. The two structures examined in the present study exhibited four response patterns to the treatments but only one pattern of response was similar in the two nuclei. The medial thalamic units are more sensitive to morphine than those recorded from the caudate nucleus. The present finding, i.e., acute effects of morphine, provides basic information with which to examine the physiological properties underlying the chronic effects of morphine.