Anaphylactoid reactions have been observed in patients treated with AN69 dialysers and ACE inhibitors. Recently, it has been shown in vitro that AN69 membranes induce the release of high amounts of bradykinin in plasma. To verify the possible role of bradykinin in these shock-like reactions, six sheep were dialysed in a random fashion using AN69 or the new SPAN membrane with and without pretreatment with captopril. All animals were dialysed for 60 min via double-lumen Shaldon catheters. Blood samples were drawn at 0, 5, 10, 15, 30, and 60 min from the venous line. A total of 24 haemodialysis procedures was carried out: group A (n = 6), AN69 without captopril; group B (n = 6), SPAN without captopril; group C (n = 6), AN69 with captopril; group D (n = 6), SPAN with captopril. A significant bradykinin release was observed only in groups A and C, reaching peak values already after 5 min. Animals in group C showed the highest bradykinin values. In four of six animals in group C anaphylactoid reactions with severe hypotension were noted. From this animal model we conclude that dialysis with the AN69 membrane is associated with bradykinin release. Pretreatment with ACE inhibitors results in further increasing bradykinin levels, which lead to anaphylactoid reactions. In contrast, the new SPAN membrane was well tolerated without detectable changes in bradykinin concentrations.