Several experimental models involving the development of cardiac hypertrophy in adult rats are characterized by the reexpression of the fetal isoform of myosin heavy chain (V3). To determine whether a similar adult-to-fetal shift in the expression of the thin-filament proteins occurs during cardiac hypertrophy, we have examined the expression of the isofonns of myosin, tropomyosin, and troponin T in the left ventricle of young spontaneously hypertensive rats (SHR) with and without treatment using enalapril, an angiotensin converting enzyme inhibitor. Phosphorylation of tropomyosin, which is predominant in the fetal state, was also analyzed. Twelve-week-old SHR were treated with enalapril for 2,5,8, and 9 weeks followed by withdrawal of treatment for 9 weeks. Control SHR, without drug treatment, were weight- and age-matched. After 9 weeks of enalapril treatment, mean arterial blood pressure was reduced (from 166±11 to 89±5 mm Hg), and left ventricular weight/body weight ratio was regressed (from 2.53±0.14 to 1.96±0.05 g/kg) to normotensive levels. During the 9-week treatment period, the percent V3decreased in SHR substantially from 35±3% to 13±1%. There was a significant correlation between the left ventricular hypertrophy and the percent V3myosin expression in the SHR during regression (r= 0.697,p< 0.001). However, only the adult isofonns of tropomyosin and troponin T were detected in the SHR with or without enalapril treatment, and the level of tropomyosin phosphorylation remained constant irrespective of the degree of left ventricular hypertrophy. These results suggest that the adult-to-fetal switch in the expression program of myosin isofonns that accompanies the development of left ventricle hypertrophy is not adopted by the thin-filament proteins, tropomyosin and troponin T.