1. 1. This research was designed to answer three questions about morphine's physiological actions. 1.1. a) Is the effect the same in three brain areas (caudate, central grey, amygdala) which are commonly believed to mediate morphine effects? 1.2. b) Does morphine have a predominantly excitatory effect on neurons in the central grey? 1.3. c) Is there diurnal variation in the physiological effects? These issues were evaluated by monitoring morphine-induced changes in unit impulse activity simultaneously from the three regions at two different times of day (0600 and 1800 hours) in 27 acutely prepared rats. 2. 2. Morphine's (15 mg/kg, i.p.) principal effect was to depress unit activity in each brain area; depression was most prominent in the caudate and central grey, being least evident in the amygdala (anterior nucleus). Naloxone (1 mg/kg) generally reversed morphine-induced depression, but had no clear effects when given after saline. No diurnal effects of morphine or naloxone were evident. Neither morphine, nor naloxone given after morphine or after saline, had any clear effect on the variance of unit activity. 3. 3. The relative lack of depression in the anterior amygdala suggests that morphine does not affect all brain areas alike, even when the areas being studied are all thought to be primarily involved in mediating morphine action. The predominance of depression in the central grey does not readily support the hypothesis that morphine \"excites\" neurons there which are inhibitory and thus block transmission of nociceptive input. Diurnal effects, if they exist at all in this species, do not seem to have any substantial magnitude in these brain areas, under these conditions. © 1978.