It is unknown whether high fetal pulmonary vascular tone is due in part to absent or decreased endothelial nitric oxide synthase (eNOS), the enzyme that produces nitric oxide in the vascular endothelium. To determine the timing of appearance and maturational changes of eNOS in the developing pulmonary circulation, we performed immunohistochemistry in lungs from fetal, neonatal, and adult sheep. Using a mouse monoclonal antibody against bovine aortic eNOS, we found immunoreactive eNOS selectively in the endothelium and it was present at all fetal ages. Immunoreactivity was seen as early as 29% gestation in the developing capillaries coursing through fetal mesenchyme. By 6 days after birth, immunoreactivity was decreased in most vessels and nearly absent in the distal pulmonary arteries of adult animals. We conclude that immunoreactive eNOS is present very early in fetal life and appears to decrease postnatally. We speculate that the early presence of eNOS in the fetal lung supports a possible role for endogenous nitric oxide activity in the regulation of vascular tone or angiogenesis in the developing pulmonary circulation.