Transferrin secretion and expression were studied in cultured Sertoli cells recovered from fats at days 10 and 17 postpartum. Transferrin biosynthesis as measured by radioimmunoassay showed a dramatic, 3.9-fold increase between days 10 and 17. The majority of the transferrin was secreted, and a kinetic study revealed that the production was four times higher at day 17 than at day 10. This difference was not the result of altered transferrin degradation as the protein was shown to be very stable at both ages. To determine if this regulation was at the transcriptional or translation level, Northern blot analysis, nuclear run-on assays, mRNA stability (half-life) measurements, and mRNA intracytoplasmic distribution analyses were carried out. The Northern blots analysis, the nuclear run-on assays, and the half-life measurements revealed that transferrin mRNA levels, gene transcription rates, and mRNA stability were indistinguishable at both ages. Interestingly, the intracytoplasmic mRNA distribution analyses showed that most of the transferrin mRNA (80%) was associated with the 40 S and 60 S protein particles at both ages and was, therefore, theoretically untranslatable. However, approximately twice as much transferrin in RNA was found to be associated with polysomes at day 17 as compared to day 10. We have shown that the increase in transferrin biosynthesis by rat Sertoli cells during testicular development is not due to an increase in the amount of transferrin mRNA or an increase in its half-life, but appears to be due to an increase in translation rate of the transferrin mRNA.