1. The opioid antagonists, naloxone, GPA 2163, levallorphan and Mr-2266 reduced the latency of the jumping reaction of mice in the hot plate test. The (+)-isomers of levallorphan and Mr-2266 which are devoid of antagonistic activity did not increase this latency. 2. In the same nociceptive reaction test, the enhancing effect of naloxone progressed in a dose-range similar to that required for the antagonism by naloxone of the depressive action of morphine. 3. The facilitatory effect of naloxone was not blocked by the previous administration of morphine or etorphine but it was prevented by pretreatment with a high dose of buprenorphine. 4. The antagonism by naloxone of morphine and of buprenorphine did not follow the same pattern. 5. The factors which are or may be involved in the efficacy of naloxone in enhancing nociceptive reactions are discussed. 6. The enhancing effect of naloxone may be due to an antagonism of endogenous ligands for the opiate receptor. If so, these ligands would be involved in reaction to but not in perception of nociceptive stimuli which need not be harmful ones.