Electroacupuncture (EA) produces suppression of the nociceptive tail flick reflex in rats. Two 10 min treatments given 90 min apart, cause a potentiation of the EA effect produced by the second treatment. This potentiation can be prevented by giving naltrexone before the first and second treatments (either intravenously or intrathecally administered). However, this potentiation cannot be reversed by naltrexone give only once, before the second EA treatment. These results suggest that the first EA effect which is opioid in nature, sets up a cascade effect, potentiating a second non-opioid EA effect.