A complementary DNA encoding the mouse epididymal secretory protein MEP 10 (mouse epididymal protein 10) was cloned and is now renamed murine epididymal retinoic acid binding protein (mE-RABP). The analysis of the predicted primary amino acid sequence showed that mE-RABP has a 75% identity with rat ESP I (epididymal secretory protein I), another epididymal retinoic acid-binding protein. The homology strongly suggests that mE-RABP is the mouse orthologue of rat ESP I. A computer analysis of the predicted three-dimensional structure confirmed that mE-RABP can accommodate retinoic acid as Ligand. In the rat, ESP I messenger RNA (mRNA) is expressed in the efferent ducts and in the entire caput epididymidis. However, in the mouse, the expression of a 950-bp mE-RABP mRNA was detected only in principal cells of the mid/distal caput epididymidis, suggesting that the regulation of region-specific expression is different in rat and mouse. Northern blot analyses showed that mE-RABP gene expression is no longer detected 10 days after castration but progressively rebounds between days 15 and 60. However, mE-RABP protein could not be detected by Western blot 30 days after castration. Androgen replacement, begun 5 days after castration and continued for 4 days restored significant expression of mF-RABP mRNA. Efferent duct ligation for 10 days did not affect gene expression. Taken together, these results indicate that mE-RABP mRNA expression is regulated by androgens but not by testicular factors. The overall similarity in the primary amino acid sequence of mE-RABP with ESP I and other members of the lipocalin superfamily suggests that they are evolutionarily related.