HMOX1 and pulmonary arterial hypertension: The main findings of this study are as follows: (a) simvastatin could ameliorate pulmonary hypertension in MCT-PH or CH-PH rats; (b) both levels and activity of HO-1 were increased in rat lung after simvastatin treatment; (c) the therapeutic effect of simvastatin was abolished by inhibiting HO-1 enzyme activity with SnPP, suggesting that the therapeutic effect of simvastatin is mediated by HO-1.