The effect of microwave irradiation on naloxone-induced withdrawal syndrome was studied in rats made morphine dependent by subcutaneous implantation of morphine pellets. Morphine-dependent rats were either exposed to pulsed low-level microwaves (2450 MHz, 1 mW/cm2, 500 pps, 2 musec pulses) or sham-irradiated for 45 min before the naloxone injection. We found that microwave-exposed rats showed significantly less weg-dog-shakes and had higher body temperature than the sham-exposed animals during withdrawal. There was no significant difference in the incident of diarrhea between the two groups of animals. These data further support the results of our previous research suggesting that pulsed low-level microwave irradiation activates endogenous opioids in the rat.