Naltrexone and the tail flick reflex.

Rats who received a tail flick test following repeated injections of 5 mg/kg of morphine were significantly more tolerant than rats who were not tested after each injection. Administration of 0.5 mg/kg of naltrexone hydrochloride, prior to the morphine injections, prevented the development of tolerance, increased analgesic latencies and abolished the difference between tested and nontested animals.