Reactive oxygen and nitrogen species have been implicated in ischemia-reperfusion (I/R) injury. Metalloporphyrins (MP) are stable catalytic antioxidants that can scavenge superoxide, hydrogen peroxide, peroxynitrite and lipid peroxyl radicals. Studies were conducted with three manganese-porphyrin (MnP) complexes with varying superoxide dimutase (SOD) and catalase catalytic activity to determine if the MnP attenuates I/R injury in isolated perfused rat livers. The release of the hepatocellular enzymes alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) was maximal at 1 min reperfusion, decreased rapidly and increased gradually by 90 min. Manganese tetrakis-(N-ethyl-2 pyridyl) porphyrin (MnTE-2-PyP) decreased ALT, AST, LDH at 1-90 min reperfusion, while manganese tetrakis-(N-methyl-2 pyridyl) porphyrin (MnTM-2-PyP) and manganese tetrakis-(ethoxycarbonyl) porphyrin (MnTECP) decreased ALT and LDH from 5 to 90 min reperfusion. The release of thiobarbituric acid-reacting substances (TBARS) was diminished by MnTE-2-PyP and MnTM-2-PyP at 90 min. The extent of protein nitration (nitrotyrosine, NT) was decreased in all three MnPs treated livers. These results demonstrate that MnP complexes can attenuate hepatic I/R injury and may have therapeutic implications in disease states involving oxidants.