The development of thin-walled cannulas which can be introduced by percutaneous technique into femoral artery and vein has revived interest in emergency cardiopulmonary support (CPS). To assess possible blood damaging effect due to the small-bore cannulas, 8 in vitro perfusions were performed with two parallel pump circuits: one circuit with a 17 F arterial and a 21 F venous cannula, one circuit without cannulas (control). The circuits were filled with 61 of bovine blood and run for 6 hours at a temperature of 37 degrees C and a flow of 4 l/min. Serial blood samples were taken before and during the experiment. There was a significantly higher level of free plasma haemoglobin (pl Hb) and a decreased number of leukocytes (Lc) after 6 hours of perfusion in the group with percutaneous cannulas as compared with control (pl Hb: 63.2 +/- 0.3 vs. 26.3 +/- 4.1 mumol/l, p < 0.05, Lc: 2.4 +/- 0.6 vs. 4.6 +/- 0.8 x 10(9)/l, p < 0.05, mean +/- se). Conclusion: percutaneous cannulas are associated with an increased blood trauma. If extended cardiopulmonary support is considered (more than several hours), percutaneous cannulas should be replaced by large-bore cannulas.