Activation of the complement system has been implicated in the pathogenesis of myocardial ischemia/reperfusion injury. It has always been assumed that liver is the primary source of complement components. In the present study, we used the reverse-transcriptase polymerase chain reaction technique to establish that the mRNAs for complement proteins C3 and C9 are expressed in rabbit heart. Rabbit liver, brain, spleen, and kidney were also shown to express C3 and C9 mRNAs. We used Western blotting to establish that these mRNAs in heart are translated into the corresponding proteins. We further established that dramatic upregulation of the mRNAs occurred in Langendorff-perfused isolated hearts subjected to ischemia and reperfusion. C3 mRNA was always expressed at higher levels than was C9 mRNA, but C9 mRNA showed greater upregulation under stress. Compared with levels in control hearts subjected to 5 minutes of normoxic perfusion, hearts subjected to 0.5 hours of ischemia followed by 1 hour of reperfusion had a 4.72-fold increase in C3 mRNA and a 19.5-fold increase in C9 mRNA. By contrast, C3 mRNA in hearts subjected to 3.5 hours of normoxic perfusion showed no change, and those subjected to 3.5 hours of ischemia showed only a 1.72-fold increase, whereas C9 mRNA levels increased by 5.17-fold after 3.5 hours of normoxic perfusion and 12.5-fold after 3.5 hours of ischemia. The results of this study demonstrate for the first time that heart tissue is capable of expressing genes and proteins of the complement system, although it is not yet known which cell types are responsible. They further demonstrate that ischemia and reperfusion of the heart promotes a rapid upregulation of the mRNAs encoding the complement proteins C3 and C9 and that these abnormal levels considerably exceed those of normal liver. These observations are consistent with the hypothesis that local production of complement proteins may contribute significantly to the degree of ischemic injury to the myocardium and that complement expression is augmented by reperfusion.