Steroid 15 alpha-hydroxylase (P450(15 alpha)) is a female-specific enzyme in the livers of many inbred mice including DBA/2J. Run-on assays using liver nuclei from GH-deficient Little mice indicated that the P450(15 alpha) gene is transcriptionally repressed by GH in male mice. BALB/cJ is a variant strain in which the gene is expressed in the males as well as in the females. Genetic crosses between DBA/2J and BALB/cJ indicated that expression of the P450(15 alpha) gene in BALB/cJ males is inherited as a recessive trait and is regulated by a single locus. The parental origin of the P450(15 alpha) genes, determined using restriction site polymorphism in the exons of the P450(15 alpha) genes, divided the F2 males expressing the P450(15 alpha) gene into three phenotypes at a ratio of 1:1:2, the individuals expressing the gene from only BALB/cJ or DBA/2J and the individuals expressing the genes from both parents respectively. The results indicate that the repression of the P450(15 alpha) gene in male mice is regulated by a trans-acting regulatory locus between the DBA/2J and BALB/cJ pairs. Because hypophysectomy derepressed the P450(15 alpha) gene in F1 males and GH repressed the gene in hypophysectomized F1 males, the hormone appears to regulate gene repression through a trans-acting locus, named GH-dependent repression, GDR.