[Changes in stimulation analgesia during chronic morphine administration].

It has been shown in rat experiments that repeated administration of morphine produces tolerance to the effect of stimulation analgesia (SA). In the control group, the same stimulation of the periaqueductal gray matter (PGM) failed to produce tolerance. Naloxone (2 mg/kg i.p.) exerted a partially antagonistic influence on SA in the control group and did not affect the antinociceptive action of PGM stimulation in morphine-treated animals.