INTRODUCTION: Cardiopulmonary bypass components need to be tested on an animal model before their clinical application. Because their weight is similar to that of man, the calf and pig are often used. This study compares the impact of prolonged perfusion on hemolysis and hematology profile in both species. METHODS: Three calves (mean bodyweight: 77.2+/-4.4 kg) and three pigs (80+/-5.3 kg) were connected to an extracorporeal circulation circuit by jugular venous and carotid arterial cannulation, with a mean flow rate of 3.5L/min for 6h. After 7 days, the animals were sacrificed. A standard battery of blood samples was taken before, throughout, and 24h, 48h and 7 days after bypass. ANOVA was used for repeated measurements. RESULTS: Absolute values of red cell count were higher in the calf (p<0.001), while normalized values were higher in the pig (p<0.001). Absolute values of white cell count were higher in the pig, while normalized values diverged toward the end of the perfusion with an increase in the calf and a decrease in the pig (p<0.001). Free plasma Hb and LDH exhibited similar profiles in both groups. CONCLUSIONS: In the setting of prolonged perfusion, species type--bovine or porcine--has an impact on hematology profile, but not on hemolytic parameters. These findings should be taken into account when cardiopulmonary bypass components are tested.