The present study evaluated the importance of ovarian functions and the renin-angiotensin system in the progression of the right ventricular (RV) hypertrophy. Female Sprague-Dawley rats were bilaterally ovariectomized (Ovx) and injected with monocrotaline (MCT, 60 mg/kg, sc). Four weeks after MCT-treatment, only the male and Ovx female rats showed marked RV hypertrophy. The hypertrophied RV of the male-MCT and Ovx-MCT rats exhibited remarkably elevated renin mRNA levels. Gene expression levels of angiotensinogen, TGF-β₁, and endothelin-1 in the hypertrophied RV also increased, but to the less degree than did the renin mRNA. To investigate beneficial effects of estrogen or enalapril on progression of the pulmonary hypertension and RV hypertrophy, histological changes of the lung and heart were examined. Sham-MCT female rats showed histological changes indicating pulmonary hypertension without RV hypertrophy. In contrast, Ovx-MCT rats showed marked RV hypertrophy with pathological changes, denoting severe pulmonary and myocardial injuries. Estrogen-or enalapril-treated Ovx-MCT rats did not show RV hypertrophy, and showed remarkably ameliorated ultrastructural changes in the lung and RV. These results from this rat model suggest that both estrogen and inhibition of the renin-angiotensin system have protective functions against the development of the pulmonary hypertension and cardiac remodeling.