Erythrocytes (RBCs) of either rabbits or healthy humans are required to demonstrate participation of nitric oxide (NO) in regulation of pulmonary vascular resistance (PVR) in the isolated rabbit lung. The property of the erythrocyte that is responsible for the stimulation of NO synthesis was reported to be the ability to release ATP in response to physiological stimuli, including deformation. Moreover, a signal transduction pathway the relates mechanical deformation of erythrocytes to ATP release has been described and the cystic fibrosis transmembrane conductance regulator (CFTR) is a component, i.e., erythrocytes of individuals with cystic fibrosis (CF) do not release ATP in response to deformation. Here we investigate the hypothesis that, in contrast to those of healthy humans, erythrocytes of humans with CF fail to stimulate endogenous NO synthesis in the isolated rabbit lung. We report that CFTR is a component of the membranes of both rabbit and human erythrocytes. Addition of the NO synthase inhibitor N^w-nitro-Larginine methyl ester (L-NAME, 100 µM) produced increases in vascular resistance in isolated rabbit lungs perfused with physiological salt solution (PSS) containing erythrocytes of healthy humans, but L-NAME was without effect when the lungs were perfused with PSS alone or PSS containing erythrocytes of CF patients. These results provide support for the hypothesis that, in cystic fibrosis, a defect in ATP release from erythrocytes could lead to decreased endogenous pulmonary NO synthesis and contribute to pulmonary hypertension.