Left ventricular hypertrophy is a compensatory response to hemodynamic overload secondary to an increased systemic resistance. This increase, however, is not the only cause of hypertrophy, and there are other factors which can have a significant effect on its incidence. To determine whether chronic antihypertensive therapy by enalapril modifies the cellular and subcellular changes of left ventricular hypertrophy observed in spontaneously hypertensive rats(SHR), 20-weeks-old SHR were treated for 22 weeks with enalapril(2mg/kg) and compared with normotensive Wister-Kyoto rats and not-treated SHR. Systolic blood pressure in enalapril-treated SHR was significantly lowered after 22 weeks compared with that of untreated control SHR group. Myocytes were reduced in size and fibrination seen in cardiac muscle fibers of control SHR was decreased in treated SHR group. Myofibrils appeared to be irregular in shape and myofilaments are decreased in control SHR but in enalapril-treated SHR the diameter and length of the myofilament became turned to regular forms. These results suggest that, enalapril, angiotensin converting enzyme inhibitor, may regress hypertrophy and some subcellular changes may be modified by enalapril.