Morphine administered intracerebrally (IC) (10 micrograms as the base on each side) into the MRF produced a significant dose dependent elevation of nociceptive threshold (i.e., analgesia) on the tail flick test and hemostat pinch test. However, morphine IC at lower doses had no analgesic effect. After morphine was injected IC (10 micrograms, bilaterally) into the MRF, naloxone, a specific narcotic antagonist, administered either IC at the same site (15 micrograms, bilaterally) or subcutaneously (10 mg/kg), antagonized the antinociceptive effects of morphine. Thirty percent of the animals given bilateral microinjection of 10 micrograms of morphine displayed hyperreactivity to mild stimuli. This hyperreactivity was not attenuated by large IC or systemic doses of naloxone. It was concluded that the MRF is a site where morphine may act to produce analgesia by a specific narcotic mechanism of action.