A major question is whether genes encoded on the sex chromosomes act directly in non-gonadal tissues to cause sex differences in development or function, or whether all sex differences in somatic tissues are induced by gonadal secretions. As part of this question we asked whether mouse X–Y homologous gene pairs are expressed in brain in a sex-specific fashion. Using RT–PCR and northern blot analysis, we assessed mRNA expression in brain of eight Y-linked genes as well as their X-linked homologues, at three ages: 13.5 days post coitum, the day of birth (P1) and adult. Transcripts of six Y genes were expressed at one or more ages: Usp9y , Ube1y , Smcy , Eif2s3y , Uty and Dby . Their expression also occurred in XY female brain, and therefore does not require testicular secretions. Six X-linked homologues ( Usp9x , Ube1x , Smcx , Eif2s3x , Utx and Dbx ) were also expressed in brain, and in adulthood all of these transcripts were expressed at significantly higher levels in brains of females than in brains of males, irrespective of their X-inactivation status. For five of these gene pairs, the expression of the Y-linked homologue in males was not sufficient to compensate for the female bias in X gene expression. Three X–Y gene pairs, Usp9x/y , Ube1x/y and Eif2s3x/y , appeared to be differentially regulated (expressed in brain in a different age- or tissue-dependent pattern), and hence may not be functionally equivalent. These sex differences in X–Y gene expression suggest several mechanisms by which these genes may participate in sex differences in brain development and function.