Aberrant RhoA/Rho-kinase signaling has been implicated in both systemic and PH. This pathway also plays important roles in a wide variety of cell functions central to embryonic and neonatal development, including proliferation, migration, and differentiation. FHR pups were raised in normoxia (sea level, 0 m) or mild hypoxia (the altitude at Denver, 1,500 m). Fasudil, a Rho-kinase inhibitor, was administered orally (30 mg/kg) from birth to rats raised in Denver. Lung tissue was collected at 1 week, 2 weeks, 3 weeks, and 4 weeks postnatally. RhoA, Rho-kinase, Rac1, and RhoE were measured by Western analysis.