We have examined the structure and expression of mRNAs encoded by the androgen-inducible RP2 gene in the kidneys of nine mouse species within the genus Mus. There is considerable interspecies variation in the lengths of the major RP2 transcripts; some of this variation is due to the presence or absence of a B1 repetitive element in the 3'-untranslated region of the gene. In addition, the extent of RP2 mRNA induction by testosterone differs among the species. Two species show 10-20-fold induction, while others display a reduced response or none at all. Analysis of an interspecific hybrid indicates that the inducibility phenotype is inherited in an additive fashion. A correlation between RP2 inducibility and the time of formation of lineages within the Mus genus suggests that induction evolved in a stepwise fashion, with the acquisition of a modest hormonal response being followed by the appearance of a greater response. The interspecies variations in RP2 mRNA structure and regulation provide a useful model for the identification and study of genetic elements that elicit evolutionary alterations in steroid-modulated gene expression.