To investigate the role of HO-1 in the pulmonary arterial pressure lowering effect of simvastatin treatment, the heme oxygenase (HO) activity competitive inhibitor, SnPP (20 μmol/kgw, s.c, on day 21~23), was administered alone or simultaneously with simvastatin to MCT-PH and CH-PH rats. This evidence concerns the gene HMOX1 and cyclic hematopoiesis.