The effects of pentobarbital anesthesia (45 mg/kg i.p.) on the inhibition of the tail-flick response induced by beta-endorphin and morphine injected intracerebroventricularly (i.c.v.) and intrathecally (i.t.) were studied in male ICR mice. Pentobarbital anesthesia attenuated the inhibition of the tail-flick response induced by morphine but not beta-endorphin given i.c.v. However, the tail-flick inhibition induced by morphine given i.t. was not attenuated by pentobarbital. beta-Endorphin-(1-27) (3 micrograms) given i.c.v. or naloxone (2 micrograms) given i.t. blocked inhibition of the tail-flick response induced by morphine given i.c.v. only in pentobarbital-anesthetized mice but not in conscious mice. beta-Funaltrexamine (beta-FNA, 2.5 micrograms) given i.c.v. or yohimbine (2 micrograms) and methysergide (2 micrograms) injected i.t. blocked the morphine (i.c.v.)-induced inhibition of the tail-flick response in conscious mice but not in pentobarbital-anesthetized mice. The results indicate that pentobarbital attenuates the morphine-induced inhibition of the tail-flick response by inhibiting descending noradrenergic and serotonergic pathways and uncovers a descending opioid system. The tail-flick inhibition induced by supraspinal morphine is mediated by stimulation of mu-opioid receptors in conscious mice and epsilon-opioid receptors in pentobarbital-anesthetized mice. The epsilon-opioid receptor-mediated descending system activated by supraspinally injected beta-endorphin is not attenuated by pentobarbital anesthesia.