Highly regionalized protein secretory activity is established progressively throughout the epididymal tubule during postnatal development. Prostaglandin D₂ synthase (PGDS) is a major protein in the ovine epididymis, the secretion of which is restricted to the proximal part of the epididymis. We investigated the mRNA and protein expression of PGDS during ontogenesis. PGDS mRNA was present in the testis and epididymis in the 50-day-old sheep fetus and persisted until 4 mo of age (2 mo before puberty) without mRNA translation in the epididymis. At 4.5 mo, mRNA was present in all of the epididymis, but translation occurred in only the proximal caput. The secreted protein present in the lumen from the caput to the cauda had the same molecular mass and isoelectric point (pI) characteristics as the testicular form. At 5.5 mo, both mRNA and protein expression were restricted to the proximal caput. The protein secreted accumulated in the cauda but was not processed after secretion as it is in the adult animal; no changes in molecular mass or pI were observed. Our results show that for at least one gene (PGDS), transcriptional and posttranscriptional regulation during postnatal development is linked to the presence of unidentified translation factors present in testicular fluid.