ACTH1-24 and beta-endorphin simultaneously injected at 5-10 microgram dose into the lateral ventricle, reciprocally suppress most of their respective behavioural effects (stretching-yawning syndrome, sexual excitement and hyperalgesia for ACTH1-24 and catalepsy and analgesia for beta-endorphin). The results obtained support the hypothesis that ACTH1-24 and beta-endorphin might interact antagonistically at CNS level.