We report on developmental changes of pulmonary and systemic nitric oxide (NO) metabolites in a baboon model of chronic lung disease with or without exposure to inhaled NO. The plasma levels of nitrite and nitrate, staining for S-nitrosothiols and 3-nitrotyrosine in the large airways, increased between 125 d and 140 d of gestation (term 185 d) in animals developing in utero. The developmental increase in NO-mediated protein modifications was not interrupted by delivery at 125 d of gestation and mechanical ventilation for 14 d, whereas plasma nitrite and nitrate levels increased in this model. Exposure to inhaled NO resulted in a further increase in plasma nitrite and nitrate and an increase in plasma S-nitrosothiol without altering lung NO synthase expression. These data demonstrate a developmental progression in levels of pulmonary NO metabolites that parallel known maturational increases in total NO synthase activity in the lung. Despite known suppression of total pulmonary NO synthase activity in the chronic lung disease model, pulmonary and systemic NO metabolite levels are higher than in the developmental control animals. Thus, a deficiency in NO production and biological function in the premature baboon was not apparent by the detection and quantification of these surrogate markers of NO production.