Comparative changes in (a) mean pulmonary artery pressure (ΔMPAP) and (b) pulmonary vascular resistance index (ΔPVRI) 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. Mean pulmonary artery pressure and PVRI were measured: (1) before NO administration (C1); (2) following seven randomized concentrations of NO between 0.15 and 150 ppm, and (3) after the cessation of NO (C2). In both groups, NO induced a significant and dose-dependent decrease in MPAP and PVRI (P< 0.01). Change in MPAP and Δ PVRI are expressed as percentage variation from the control value. In both groups, a plateau effect was observed for MPAP and PVRI from NO concentrations of 4.5 ppm. No interaction between the factors `group' and `does of NO' was found using the two-way analysis of variance, suggesting that the NO dose-response was not affected by the presence of septic shock.