Large samples of original cDNAs encoding neonatal and adult mouse fast skeletal muscle troponin T (fTnT) have been isolated and characterized. The results demonstrate expression relationships of 8 alternatively spliced exons of the fTnT gene and reveal the primary structure of as many as 13 fTnT isoforms that diverge into acidic and basic classes due to differential mRNA splicing in the N-terminal variable region. In the C-terminal variable region encoded by the mutually exclusive exons 16 and 17, the splicing pathway and structure of exon 16 appears to be adult fTnT-specific, suggesting an adaptation to the functional demands of mature fast skeletal muscle. The cloned cDNAs were expressed in E. coli as standards to identify a high M(r) to low M(r), acidic to basic fTnT isoform transition in postnatal developing skeletal muscles. Different from the developmental cardiac TnT switch generated by alternative splicing of a single exon, the fTnT isoform transition is an additive effect of alternative splicing of multiple N-terminal-coding exons, especially exons 4, 8 and fetal that are expressed at higher frequencies in the neonatal than in the adult muscle. The developmental fTnT isoform primary structure transition in both N- and C-terminal variable regions suggest a physiological importance of the apparently complex TnT isoform expression.