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We have cloned cDNA sequences of both rat and mouse slow troponin T gene. These sequences share a high level of homology with each other and with the human slow troponin T gene although we were unable to detect an alternatively spliced exon present at 3′ end of human slow troponin T cDNA in either mouse or rat cDNAs. Northern blot analysis detected a high level expression of slow troponin T in adult mouse Soleus with a lower level expression in mixed postnatal skeletal muscles. Unlike late fetal and postnatal skeletal muscles in which slow troponin T expression is restricted to slow muscle fibre rich regions only, in situ hybridisation analysis detected this isoform to be highly expressed in somitic myotome and all muscle masses at 10–14 days of gestation after which its expression was rapidly downregulated. The unexpected expression of slow troponin T mRNA in fetal heart was apparent by both northern blotting and in situ hybridisation analyses. Slow troponin T mRNA in fetal heart was first detected at 10 day in utero reaching maximum levels of expression at 12–15 days gestation. The slow troponin T in the heart was mainly expressed in the ventral ventricles until day 15 after which low level expression was also observed in both atria. Slow troponin T mRNA in both atrium and ventricle was mainly expressed in outer wall of the myocardium although it was also expressed in interventricular septum. This study therefore shows that in addition to being a cell type specific marker during later fetal and postnatal skeletal muscle development, slow troponin T represented one of the major developmental isoforms expressed in embryonic and fetal skeletal muscle as well as in the cardiac muscle.