BACKGROUND AND OBJECTIVES: Cardiac troponin T (cTnT) has been used as a very sensitive marker of cardiac injury caused by ischaemia, myocarditis, and cardiomyopathy. After cardiac injury, the fetal cTnT isoform expression in the heart and serum cTnT increases. To investigate the increased levels of serum cTnT, and the expression of fetal cTnT isoform in the heart, that can predict myocardial injury, we measured serum cTnT levels and the fetal cTnT isoform expression at various time points during the early phase of myocardial toxicity induced by adriamycin (ADR) in rat. MATERIALS AND METHODS: Male Sprague-Dawley rats were injected, intraperitoneally, with ADR (5 mg/kg) twice a week for 2 weeks. Control rats were injected with saline. Serum cTnT levels were measured by ELISA. The ratio of fetal/adult (F/A) cTnT isoform expression (%) was semi-quantified by RT-PCR using total RNA from frozen hearts. RESULTS: Serum cTnT levels did not increase by 1 week after ADR injection, but increased significantly after 2 weeks. The ratio of F/A cTnT in the heart significantly increased from day 1, peaked at 1 week and persisted until the end of 2 week. CONCLUSION: The expression of the fetal cTnT isoform occurred from 1 day after ADR injection when the serum cTnT levels were still normal. Although the serum cTnT level is a very sensitive, and an early marker, of cardiac damages, the fetal cTnT isoform expression in the endomyocardial biopsy specimen may be a more sensitive and an earlier marker in the ADR-induced myocardial damage.