Objective(s): To study the effect of cardiopulmonary bypass (CPB) on nuclear factor-kappa B (NF-кB) and cytokine expression and pulmonary function in dogs. Materials and Methods: Twelve male mongrel dogs were divided into a methylprednisolone group (group M) and a control group (group C). All animals underwent aortic and right atrial catheterization under general anesthesia. Changes in pulmonary function and hemodynamics were monitored and the injured site was histologically evaluated. Results: The activity of NF-кB and myeloperoxidase (MPO), levels of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and IL-8, and the wet/dry (W/D) weight ratio were significantly higher after CPB than before CPB in both groups (P