Little is known about the physiological roles of the M 5 muscarinic receptor, the last member of the muscarinic receptor family (M 1 –M 5 ) to be cloned. In the brain, the M 5 receptor subtype is preferentially expressed by dopaminergic neurons of the substantia nigra and the ventral tegmental area. Dopaminergic neurons located in the ventral tegmental area are known to play important roles in mediating both the rewarding effects of opiates and other drugs of abuse and the manifestations of opiate/drug withdrawal symptoms. We therefore speculated that acetylcholine-dependent activation of M 5 receptors might modulate the manifestations of opiate reward and withdrawal. This hypothesis was tested in a series of behavioral, biochemical, and neurochemical studies using M 5 receptor-deficient mice (M 5 −/− mice) as novel experimental tools. We found that the rewarding effects of morphine, as measured in the conditioned place preference paradigm, were substantially reduced in M 5 −/− mice. Furthermore, both the somatic and affective components of naloxone-induced morphine withdrawal symptoms were significantly attenuated in M 5 −/− mice. In contrast, the analgesic efficacy of morphine and the development of tolerance to the analgesic effects of morphine remained unaltered by the lack of M 5 receptors. The finding that M 5 receptor activity modulates both morphine reward and withdrawal processes suggests that M 5 receptors may represent a novel target for the treatment of opiate addiction.