Capillary density, as quantified by the number of barium-filled pulmonary arteries counted per high-power field, was significantly decreased in hyperoxia-exposed lungs compared with room air controls. Treatment with L-citrulline preserved lung capillary density in experimental BPD. N=5/group, §P<0.0001 hyperoxia vs normoxia and normoxia+L-Citrulline, **P<0.001 hyperoxia vs hyperoxia+L-Citrulline, *P<0.02 normoxia+L-Citrulline vs hyperoxia+L-Citrulline.