The aim of this study was to investigate the possible participation of nitric oxide in the agmatine-mediated potentiation of morphine-induced analgesia in mice. Agmatine and L-NAME (a nitric oxide synthesis inhibitor) enhanced morphine-induced analgesia in the tail flick test, but not in the hot plate test. L-NAME did not block the agmatine-induced potentiation of morphine effect. Our results indicate that agmatine potentiates morphine-induced spinal but not supraspinal analgesia, and that this effect is not mediated by a nitric oxide-dependent mechanism.