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Abstract We tested the claim that the dopaminergic dysfunction of Rett Syndrome (RTT) also occurs in Mecp2-deficient mice that serve as a model of the syndrome. We used positron emission tomography (PET) to image dopamine D 2 receptors (D 2 R) and transporters (DAT) in women with RTT and in Mecp2-deficient mice, and D 1 R and D 2 R density was measured in postmortem human tissue by autoradiography. Results showed 1) significantly reduced D 2 R density in the striatum of women with RTT compared to control subjects. 2) PET imaging of mouse striatum similarly demonstrated significant reductions in D 2 R density of 7–10 week-old hemizygous ( Mecp2 -null) and heterozygous (HET) mice compared to wild type (WT) mice. With age, the density of D 2 R declined in WT mice but not HET mice. 3) In contrast, postmortem autoradiography revealed no group differences in the density of D 1 R and D 2 R in the caudate and putamen of RTT versus normal control subjects. 4) In humans and in the mouse model, PET revealed only marginal group differences in DAT. The results confirm that dopaminergic dysfunction in RTT is also present in Mecp2-deficient mice and that reductions in D 2 R more likely explain the impaired ambulation and progressive rigidity observed rather than alterations in DAT. Highlights • PET imaging of dopamine function was performed in MeCP2-deficient humans and mice. • D 2 receptor binding is reduced in Rett syndrome and in Mecp2-deficient mice. • Dopamine transporters are less affected than D 2 receptors by MeCP2 deficiency. • Mecp2-deficient mice model dopamine dysfunction in Rett Syndrome • D 2 receptors may play a role in motor deficits in Rett Syndrome. [ABSTRACT FROM AUTHOR]