Changes in (a) PaO2/FiO2 (Δ PaO2/FiO2 and (b) venous admixture (QVA/QT) induced by increasing inspiratory intratracheal concentrations of inhaled NO (Insp IT NO) in the presence (n = 8, ●) or absence (n = 8, ○) of septic shock in 16 patients with ARDS. PaO2/FiO2 and QVA/QT were measured: (1) before NO administration (C1); (2) following seven randomized concentrations of NO between 0.15 and 150 ppm, and (3) after cessation of NO (C2). Δ PaO2/FiO2 and QVA/QT are expressed as percentage variation from the control value. In both groups, NO induced a significant and dose–dependent increase in PaO2/FiO2 and a decrease in QVA/QT (P< 0.01). In both groups, a plateau effect was observed for the NO-induced decrease in QVA/QT from NO concentrations of 1.5 ppm. In patients with septic shock, NO-induced increases in PaO2 did not show any plateau whereas in patients without septic shock a plateau effect was observed from NO concentrations of 4.5 ppm. An interaction between the factors 'group' and 'dose of NO' was found using the two-way analysis of variance (P = 0.035) suggesting that the profile of the NO dose–response curve was affected by the presence of septic shock.