Total RNA from mouse epididymides was translated in a cell-free system derived from rabbit reticulocyte lysate. The androgen dependence of a highly represented mRNA(s) was detected. This mRNA(s) encoded for a band of Mr 26,000 visualized by denaturing gel electrophoresis (SDS-PAGE). No other mouse sexual tissues (testis, vas deferens and seminal vesicle), liver, kidney or striated muscle presented such a band. Furthermore, this mRNA(s) was restricted to the caput epididymidis. Two-dimensional gel electrophoresis (2D-PAGE) showed that this band of Mr 26,000 was composed of 6 basic polypeptides. This translated protein may correspond to a newly synthesized secretory protein of Mr 24,000. The concentration of translatable mRNA(s) encoding for the band of Mr 26,000 dropped to 31% of the normal level at 3 days after castration and to 7%, 20 days later. Administration of testosterone to 30-day-castrated mice partly reversed these changes: 32% of the control value was reached after 10 days of treatment. Accumulation of this mRNA(s) was also just detectable (7% of the mature level) at 10 days of age. The high levels attained by 20 days of age (60% of the control value) suggest that the greatest increase of accumulation for this mRNA(s) occurs during the prepubertal period.