In order to study the cardioprotective effects of vanadate on ischemia-reperfusion (I/R) injury, isolated rat hearts perfused at a constant flow were subjected to global ischemia for 30 min followed by reperfusion for 30 min. In this experimental model, I/R was observed to produce a marked decrease in ventricular developed pressure and an increase in end diastolic pressure. Pretreatment of hearts with 4 µM vanadate was found to attenuate I/R-induced cardiac dysfunction. The reduction in sarcoplasmic reticulum (SR) Ca²⁺-uptake and Ca²⁺-release activities as well as SR protein contents for Ca²⁺-pump ATPase and Ca²⁺-release channel was also prevented by vanadate. Pretreatment of hearts with an antioxidant mixture containing superoxide dismutase plus catalase (SOD + CAT) was found to exert effects similar to those seen with vanadate in I/R hearts. Post-ischemic treatment of hearts with vanadate or SOD + CAT also produced beneficial actions on I/R-induced changes in cardiac performance and SR function. Alterations in cardiac function and SR Ca²⁺-transport activities due to an oxyradical generating system (xanthine plus xanthine oxidase) or an oxidant (hydrogen peroxide) were attenuated by treatment with vanadate. These results suggest that vanadate may exert beneficial effects on cardiac performance and SR function in I/R hearts due to its antioxidant action.