Several X-linked neurodevelopmental disorders including Rett Syndrome, induced by mutations in the MECP2 gene, and Fragile X Syndrome (FXS), caused by mutations in the FMR1 gene, share autism-related features. The mRNA coding for Methyl CpG binding protein 2 (MeCP2) has previously been identified as a substrate for the mRNA-binding protein Fragile X Mental Retardation Protein (FMRP), which is silenced in FXS. Here, we report a homeostatic relationship between these two key regulators of gene expression in mouse models of FXS (Fmr1 KO) and Rett syndrome (MeCP2 KO). We found that the level of MeCP2 protein in the cerebral cortex was elevated in Fmr1 KO mice, whereas MeCP2 KO mice displayed reduced levels of FMRP, implicating interplay between the activities of MeCP2 and FMRP. Indeed, knockdown of MeCP2 with short hairpin RNAs led to a reduction of FMRP in mouse Neuro2A and in human HEK-293 cells, suggesting a reciprocal coupling in the expression level of these two regulatory proteins. Intra-cerebroventricular injection of an adeno-associated viral vector coding for FMRP (AAV-FMRP) led to a concomitant reduction in MeCP2 expression in vivo, and partially corrected locomotor hyperactivity. Additionally, the level of MeCP2 in the posterior cortex correlated with the severity of the hyperactive phenotype in Fmr1 KO mice. These results demonstrate that MeCP2 and FMRP operate within a previously undefined homeostatic relationship. Our findings also suggest that MeCP2 overexpression in Fmr1 KO mouse posterior cerebral cortex may contribute to the fragile X locomotor hyperactivity phenotype.© The Author(s) 2020. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: email@example.com.