Injection of protein-A purified antibodies against Met-enkephalin and beta-endorphin into the periaqueductal gray matter (PAG) was shown to decrease the analgesic effect of electroacupuncture (EA) in rabbits. Met-enkephalin antibodies were more potent than the beta-endorphin antibodies in causing a statistically-significant effect on electroacupuncture analgesia. Antibodies to Met-enkephalin were also active at the spinal level, whereas antibodies against beta-endorphin were without effect: this is in agreement with a rich enkephalinergic innervation and absence of beta-endorphin-containing fibres in the spinal cord. Substance P, the other neuropeptide of this study, also seems to be important in mediating effects of electroacupuncture. Injection of antibodies into the periaqueductal gray caused decrease of the effect of electroacupuncture whereas intrathecal administration of Fab-fragment substance P antibodies caused a marked potentiation. The demonstration of site specificity of the neuropeptides in mediating analgesia induced by electroacupuncture supports the validity of this experimental approach.