The importance of Cocaine- and amphetamine-regulated transcript (CART) peptide in reinforcing effects of addictive drugs specially alcohol and psychostimulants has been stablished. Involvement of CART peptide in rewarding effects of opioids in brain has recently been reported. Here we have studied the expression of CART mRNA and peptide in the reward pathway in morphine-induced sensitization phenomenon and also evaluated the peptide level fluctuations in CSF and plasma. Male Wistar rats received 7-day morphine injection (20 mg/kg) and then after a 7-day washout period, a challenge dose of 10 mg/kg morphine was administered and locomotor activity and oral stereotypical behaviors were recorded. Besides, the expression level of CART mRNA and peptide in four important areas of the mesocorticolimbic reward pathway including nucleus accumbens, striatum, prefrontal cortex, and hippocampus were measured by real-time PCR and western blotting, respectively. The level of the peptide in CSF and plasma was measured by Elisa method. The expression level of CART mRNA and protein in brain regions and also the peptide level in CSF and plasma were significantly down-regulated after 7-day morphine administration. These reduced levels returned to nearly normal rates after 7-day wash-out period. Administration of morphine challenge dose led to significant upregulation of CART gene expression (both mRNA and peptide) in the brain, and elevation of peptide level in CSF and plasma in morphine-sensitized rats. It can be concluded that CART is released in the framework of reward pathway and may serve as an important neurotransmitter in the process of morphine dependence and sensitization.