Rats were anesthetized with a continuous infusion of pentobarbital. First we showed that electroacupuncture (EA) suppressed the tail flick nociceptive reflex. Naltrexone (a long acting opiate antagonist) was given either intravenously or intrathecally to attempt to block this EA effect. The naltrexone prevented this EA effect when injected before the onset of EA treatment, but failed to reverse the EA effects when injected immediately after termination of EA treatment. We discuss the possibility that the endorphins set up a cascade effect whereby the subsequent EA suppression of tail flick is no longer dependent on endorphin and hence cannot be affected by opiate antagonists. This finding may clarify some of the confusion in the acupuncture endorphin literature.