Whole-body y-irradiation(WBI), which produces an oxidative stress, is reported to attenuate the acute antinociceptive action of morphine (aμ-opioid receptor agonist), but not DPLPE (að-opioid receptor agonist), in mice. Recently, we also reported that antinociceptive effect of morphine, but not β-endorphin (a novel ε-opioid receptor agonist), was attenuated by oxidative stress. These findings prompted us to investigate the effect of WBI on the antinociception of morphine and β-endorphin in mice. Mice were exposed to WBI (5 Gy) from a 60Co gamma-source and tested 2 hours later for antinociception produced by intracerebroventricular administration of morphine or β-endorphin using the hot water tail-immersion and the writhing tests. WBI significantly attenuated the antinociception produced by morphine only in the hot water tail-immersion test, whereas the antinociception of -endorphin was significantly potentiated by WBI in both tests. These results demonstrate a differential sensitivity of μ- and ε-opioid receptors to WBI, and support the hypothesis that morphine and β-endorphin administered supraspinally produce antinociception by different neuronal mechanisms.