Dmrt1 is a recently described gene that is expressed exclusively in the testis and is required for postnatal testis differentiation. Here we describe the expression of Dmrt1 in postnatal rat testis and Sertoli cells. RNase protection analysis was used to examine Dmrt1 mes- senger RNA (mRNA) levels in intact testis during postnatal devel- opment and in primary cultures of Sertoli cells under various culture conditions. We show that Dmrt1 mRNA levels rise significantly be- ginning approximately 10 days after birth and remain elevated until after the third postnatal week. Thereafter, mRNA levels drop coin- cident with the proliferation of germ cells in the testis. In freshly isolated Sertoli cells, Dmrt1 mRNA levels were robust but decreased significantly when the cells were placed in culture for 24 h. Treatment of Sertoli cells with either FSH or 8-bromo-cAMP resulted in a sig- nificant rise in Dmrt1 mRNA levels. This cAMP response was sen- sitive to treatment with the transcriptional inhibitor actinomycin D but not to the translational inhibitor cycloheximide. The cAMP- dependent rise in Dmrt1 mRNA also required activation of protein kinase A, as mRNA induction was sensitive to the inhibitor H89. Studies also show that Dmrt1 expression was inhibited by phorbol esters (PMA) but only modestly effected by serum. (Endocrinology 142: 1167-1178, 2001)