Background: Simulated extracorporeal circulation (SECC) induces inflammatory reaction. Nitric oxide (NO) has pro-and anti-inflammatory properties. NO role in SECC-related inflammatory response is unclear. The aim of this study was to clarify if NO affects the foreign-surface induced leukocyte activation during SECC. Methods: Human blood was circulated through SECC during 3 hours. Control group C was ventilated with oxygen/air mixture and the study group with oxygen/air mixture and NO. Leukocyte activation was measured as serum levels of myeloperoxidase (MPO), human neutrophil lipocalin (HNL), lactoferrin (LF), interleukin-1-beta (IL-1β) and interleukin-10 (IL-10). Oxygen free radical production capacity was evaluated with chemiluminescence. NO metabolites nitrite/nitrate were estimated in serum. Results: Leukocyte granule release increased over time. Addition of NO significantly increased MPO, HNL and LF release. The average difference increased with SECC duration. NO addition did not significantly affect measured interleukins concentration or oxygen free radical production capacity. NO metabolites increased significantly in the NO circuits. Conclusions: Results indicate that NO addition during SECC is pro-inflammatory and has no effect on oxygen free radical production and interleukin release.