Troponin I is the inhibitory component of troponin, the thin filament regulatory complex in striated muscle. Separate genes encode cardiac-specific fast and slow skeletal-specific isoforms of this protein. We have previously described gene switching from the slow skeletal to the cardiac troponin I mRNA expression in developing rat heart. The purpose of this work was to characterize the expression of the different troponin isoforms in the human heart. Human cardiac and slow skeletal troponin I cDNA probes were obtained by screening an adult cardiac cDNA library and by Taq polymerase amplification of RNA from an infant's heart, respectively. We found that the cardiac troponin I isoform is tissue-specific in its expression in normal adult tissues. RNA blot analysis of cardiac ventricular RNA from infants with congenital heart disease and from an adult with cardiomyopathy revealed expression of human cardiac troponin I in all analyzed specimens. In addition, we found expression of slow skeletal troponin I mRNA and protein in infant hearts but no detectable mRNA expression in the adult heart. We conclude that troponin I isoforms are developmentally regulated in the human heart by a mechanism similar to that in the rat heart.