Reward mechanisms are involved in pair bond formation in monogamous prairie voles. Given the potential role of CART (cocaine- and amphetamine-regulated transcript) in reward, and its possible role as a third neurohypophysial hormone, we examined the brain distribution of CART mRNA and peptide in monogamous prairie voles compared to congener promiscuous meadow voles. Large species differences in CART mRNA distribution were apparent in the nucleus accumbens, bed nucleus of the stria terminalis, hippocampus, and cortex. CART peptide distribution largely mirrored, but did not exactly match, CART mRNA distribution. Dramatic species differences also existed in CART peptide distribution, including the medial preoptic area, nucleus accumbens, central amygdala, lateral septum, and cortex. In contrast, several brain regions were highly conserved between prairie and meadow voles, including many subnuclei examined within the hypothalamus and olfactory tubercle. Taken together, these data suggest a potential role for CART in the regulation of pair bond formation between monogamous mates and suggest potential brain regions involved in its neural circuitry. Our findings also point to novel avenues of investigation regarding the brain mechanisms for the evolution of diverse social organization.