Early spermatids contain roughly 1000-fold more TATA-binding protein (TBP) mRNA than do somatic cells. The appearance of TBP-overexpressing spermatids in the developing testis is accompanied by a large increase in whole-organ levels of total RNA and of poly(A)+ RNA per cell. Whereas somatic cells initiate transcription of TBP mRNA at a single promoter/first exon (exon 1C), in adult testis, two additional major promoter/first exons (1D and 1E) are used. We have examined the expression of the somatic and testis-specific TBP mRNA isoforms during rodent testis development. In juvenile testes TBP mRNAs containing either exon 1C or exon 1D, but none containing exon 1E, are detected. At 21 days of age, all TBP mRNA isoforms begin to overaccumulate. The onset of TBP mRNA overaccumulation is marked first by an increase in levels of polysomal TBP mRNA, and later by accumulation of mRNP-associated TBP mRNA. In adult testes, only 30% of the total TBP mRNA is engaged by polysomes; the remainder is sequestered as mRNP particles. All of the TBP mRNA isoforms in adults exist both as free mRNP particles and as polysomes; however, the fraction in polysomes varies from 60% (exon 1C) to 10% (exon 1E). This suggests that sequences within the first exons alter the probability that the mRNA will either assemble into polysomes or into translationally inactive mRNP particles.