Cocaine- and amphetamine-regulated transcript (CART) peptides are found in brain and spinal cord areas involved in pain transmission. In the present study, we investigated the role of rat CART (55-102) in the modulation of an acute pain model after intrathecal administration. The results show that CART (55-102) was without effect on the tail-flick test after i.t. injection in mice. Interestingly, i.t. administration of CART (55-102) significantly enhanced the antinociceptive effect of morphine in the tail-flick test. These results suggest that CART (55-102) potentiates the effects of opioids to inhibit the nociceptive information transmission.