A recombinant cDNA plasmid derived from mouse urinary protein (MUP) mRNA was isolated and used to determine the level of control of the developmentally regulated and the sex-linked expression of MUP genes by monitoring the transcription of MUP mRNA sequences in isolated liver nuclei. No transcription of MUP genes could be detected in liver nuclei of prepubescent animals whose livers do not contain measurable MUP mRNA. Transcription of MUP genes in the livers of adult male mice was 6-fold higher than in the livers of adult female mice, proportional to the difference in MUP mRNA concentrations. Transcriptional control mechanisms are therefore implicated as responsible for both the developmentally and the sex-linked changes in the expression of MUP genes.