Objective To investigate the fluctuations in arterial leukocyte phospholipase D (PLD) activity during the perioperative period of open heart surgery under cardiopulmonary bypass (CPB), and the relationship between PLD activity and systemic inflammatory response induced by CPB.Methods Arterial blood was obtained from 26 patients undergoing open heart surgery at 8 different time points during the perioperative period, from which leukocytes were isolated for determination of PLD activity, CD11b expression and myeloperoxidase (MPO) activity. Plasma IL-6, IL-8 and C-reactive protein were also determined. The 26 cases were retrospectively divided into 3 groups according to perfusion time in order to detect the possible influences of CPB on PLD activity and IL-6 and IL-8 levels.Results When the ascending aorta was declamped, average arterial leukocyte PLD activity was 0.305±0.132 nmol choline·min-1·mg-1，5.0 times higher of the pre-CPB value, and remained (5.4 times higher of the pre-CPB level) at 72 hours after CPB. Leukocyte CD11b expression and plasma IL-6 and IL-8 levels increased significantly at the end of CPB, while MPO activity and C-reactive protein concentration reached their peaks at 1 and 24 hours, respectively, after CPB. At the end of CPB, the arterial leukocyte PLD activity of patients whose CPB duration was longer than 90 minutes were 1.82- and 1.74-fold that of the other two groups with CPB lasting between 90 and 60 minutes and less than 60 minutes.Conclusions Arterial leukocyte PLD activity rises significantly in CPB and its elevation is earlier and more persistent than other inflammation-related indicators tested; longer CPB duration leads to higher leukocyte PLD activity at the end of CPB. These results imply that PLD could be a new target for prevention of systemic inflammatory response induced by CPB.