The antioxidant effects of ceruloplasmin (CAS 9031-37-2) against oxygen free radicals (.O2-, .OH, 1O2) and their by-products (H2O2, HOCl), generated by electrolysis of Krebs-Henseleit buffer, were determined in vitro by the DPD (N,N-diethyl-p-phenylenediamine) colorimetric method and ex vivo by quantifying cardiodynamic variables of the isolated perfused rat heart. Purified ceruloplasmin (1 mumol/l) displayed a high antioxidant capacity in vitro (89.2%), while the scavenging capacity of superoxide dismutase (SOD) in equimolar concentrations was 38.1%. A relatively high scavenging activity (72.1%) was observed with bovine serum albumin (BSA). A control group of Langendorff isolated rat hearts (n = 8) was submitted to electrolysis (10 mA, for 1 min) without treatment, whereas the treated groups were perfused with ceruloplasmin, SOD or BSA (1 mumol/l) in the inflow cannula for 5 min before, during, and 5 min after electrolysis. The cardioprotective effect afforded by ceruloplasmin (83-89%) was higher than that observed with the same optimal dose of 1 mumol/l SOD (20-45%). With BSA, no protection was observed ex vivo. Particularities in scavenging specificities and mechanisms seem to explain the important differences between in vitro and ex vivo antioxidant capacities for these proteins.