Naloxone injected intrathecally had a different effect on the inhibition of the inhibition of the tail flick response produced by beta-endorphin and morphine injected intraventricularly. The intrathecal naloxone completely antagonized the effect of beta-endorphin but had only a very weak effect on morphine. In the same rats the cataleptic response to beta-endorphin was antagonized as well; however no definitive conclusion could be made regarding the antagonism of the morphine-induced catalepsy. The results indicate that spinal endorphin is involved in the production of intraventricular beta-endorphin-induced spinal tail flick inhibition and suggest that intraventricular beta-endorphin and morphine elicit their pharmacological action via the activation of different descending inhibitory systems.