The effects of a wide range of morphine doses and of its antagonist, naloxone, on spontaneous multiunit discharges in freely moving rats were recorded simultaneously from the septum (Spt), medial thalamus (CM-PF complex), and caudate nucleus (CN). A high percentage of neurons in these three areas are affected by morphine. Neurons in the CM-PF complex exhibited a greater number of morphine-induced changes (104/145) than did those in the caudate nucleus (79/160), or in the septum (67/150). The morphine-induced changes exhibited dose-related patterns: the three structures examined in the present study exhibited four response patterns to incremental doses of morphine: either a monophasic effect, an increase or decrease in firing rate, or a biphasic effect; ie, lower morphine doses induced a decrease in activity, whereas higher doses induced an increase in firing rate. There was no observed correlation between the response patterns in the three regions. The technique provides a tool with which to identify and classify the specific response patterns induced by morphine in specific brain regions, and the results may indicate that each region plays a different physiological role in the effects induced by morphine.