Various inflammatory mediators are activated by triggers such as surgical injury at the time of heart surgery, endotoxemia, ischemia, and the contact of the blood in the CPB circuit surface. The inflammatory response of the whole body by the inflammation mediator causes postoperative renal failure, respiratory failure, hemorrhage, nerve damage, cardiac arrhythmia, myocardial disorder, and it progresses to serious multi-organ failure. The objective was to clarify whether inflammatory mediators such as IL-6, IL-10, and TNF-α after open heart surgery using cardiopulmonary bypass (CPB) are activated in dogs. Open heart surgery was performed in 9 dogs (BW3.0-7.3 kg) using CPB from April 2007 to December 2007. The plasma cytokine level was measured preoperatively at 5 minutes after heparin administration, 5 minutes after CPB, after rebeating of the heart, 15 minutes after protamine administration, and after weaning of CPB at 3, 6, 12, 24, and 48 hours. IL-6 was elevated from preoperatively (1.3 ± 2.4 pg/ml) after 3 hours weaning of CPB (281 ± 189 pg/ml). IL-10 was elevated from preoperatively (2.6 ± 4.2 pg/ml) to 15 minutes after protamine (54.2 ± 47.5 pg/ml). There was no difference in TNF-α in all operative periods. Inflammatory cytokine was elevated in dogs in which cardiac surgery had been performed using CPB. This suggests the dogs were in an acute inflammation condition after surgery. It is postulated that the elevation of the inflammatory cytokine develops into a postoperative systemic inflammatory response syndrome.