OBJECTIVES: A growing body of evidence suggests that age affects the main pathophysiologic mechanisms of the acute respiratory distress syndrome. This may imply the need for developing age-tailored therapies for acute respiratory distress syndrome. However, underlying molecular mechanisms governing age-related susceptibility first need to be unraveled. In a rat model of acute lung injury, we investigated whether age affects the balance between the two key enzymes of the pulmonary renin-angiotensin system, angiotensin-converting enzyme, and angiotensin-converting enzyme 2. We hypothesized that aging shifts the balance toward the lung injury-promoting angiotensin-converting enzyme, which may form an explanation for the differences in severity of lung injury between different age groups. DESIGN: Prospective, randomized controlled animal study. SETTING: University medical research laboratory. SUBJECTS: Infant (15 ± 2 d), juvenile (37 ± 2 d), adult (4 ± 0.2 mo), and elderly (19.5 ± 0.5 mo) male RCCHan Wistar rats. INTERVENTIONS: Lung injury was induced by intratracheal administration of lipopolysaccharide (5 mg/kg) and 4 hours of mechanical ventilation (15 mL/kg). MEASUREMENTS AND MAIN RESULTS: In lipopolysaccharide-exposed and mechanical ventilated rats, angiotensin-converting enzyme activity in bronchoalveolar lavage fluid increased 3.2-fold in elderly when compared with infants. No changes in bronchoalveolar lavage fluid angiotensin-converting enzyme 2 activity were found. In addition, membrane-bound angiotensin-converting enzyme activity decreased. Together with the presence of angiotensin-converting enzyme-sheddase ADAM9 (a disintegrin and metalloproteinase domain-containing protein 9) and an age-dependent increase in tumor necrosis factor-α, an activator of ADAM9, these results indicate increased shedding of angiotensin-converting enzyme in the alveolar compartment, thereby shifting the balance toward the injurious pathway. This imbalance was associated with an increased inflammatory mediator response and more lung injury (wet-to-dry ratio and histology) in elderly rats. CONCLUSIONS: Increasing age is associated with an imbalance of the pulmonary renin-angiotensin system, which correlates with aggravated inflammation and more lung injury. These changes might form the ground for new therapeutic strategies in terms of dosing and effectiveness of renin-angiotensin system-modulating agents for treatment of acute respiratory distress syndrome.