The analgesic effect produced by subcutaneous injection (SC) of morphine was antagonized by intracerebroventricular (ICV) but not intrathecal (ITH) injection of CaCl2. While ITH CaCl2 was devoid of any effect on the analgesia induced by ITH morphine, it did antagonize the analgesic effect produced by ITH injection of dynorphin A or (D-Pen2,D-Pen5)-enkephalin (DPDPE). In accordance with this, the uptake of 45Ca by synaptosomes prepared from the dorsal column of rat spinal cord was suppressed by dynorphin A or DPDPE, but not by morphine. The results imply that different kinds of opioid ligands might have different mechanisms of action at discrete areas of the CNS. Blockade of neuronal Ca2+ uptake may serve as an important mechanism for morphine analgesia in the brain as well as DPDPE and dynorphin A analgesia in the spinal cord.