Cardiopulmonary bypass (CPB) preserves patients’ lives during open heart surgery by providing sufficient oxygen delivery and blood supply to vital organs. However, previous studies have suggested that the interaction of hemodilution and vascular hyperpermeability induces tissue edema and an inflammatory response during CPB. In this study, we hypothesized the suppression of the systemic inflammatory response and tissue edema during CPB by a plasma substitute (hydroxyethyl starch [HES]). Rats (450–500 g) were divided into a SHAM group (n = 5), a Ringer’s acetate CPB group (n = 7), and an HES CPB group (n = 7). In the Ringer’s acetate group, the CPB circuit was primed with Ringer’s acetate solution, and in the HES CPB group, it was primed with HES formulation (6% HES 130/0.4). Blood samples were collected before (baseline) and 30, 60, 90 and 120 min after initiation of CPB. Plasma cytokine levels of tumor necrosis factor-α, interleukin (IL)-6, and IL-10, and biochemical markers (lactate dehydrogenase, aspartate aminotransferase, alanine aminotransferase, blood urea nitrogen, creatinine, liver-type fatty acid–binding protein, and colloid osmotic pressure [COP]) were measured before and 30, 60, 90, and 120 min after the initiation of CPB. In the Ringer’s acetate CPB group, the inflammatory cytokines and biochemical markers increased significantly during CPB compared with the SHAM group, but such increases were significantly suppressed in the HES CPB group. In addition, during CPB, it was possible to preserve normal plasma COP in the HES CPB group. The data suggest that 6% HES 130/0.4 is effective for suppressing the inflammatory response during CPB.