The use of heterotopic cardiac transplantation in rats of isogeneic strain has been investigated as a model of diffuse myocardial ischaemic injury. This model allowed the production of controlled graded injury prior to implantation of the donor heart in a recipient animal. In 21 animals the uptake of technetium-99m labelled pyrophosphate ([99mTc]PYP) in the transplanted heart, as measured by a γ-camera/computer system, agreed well with that determined by direct organ counting in a sodium iodide well-type detector (P < 0.001). The maximum uptake of [99mTc]PYP in the transplanted heart, as determined using the γ-camera, was found to be at 2 days after transplantation and 1 to 2 h after injection. For three groups each consisting of 25 rats the donor hearts were stored for 0, 30 or 90 min to produce a range of myocardial damage. For these groups there was no significant increase in creatine kinase with increasing storage time. For each group the uptake of [99mTc]PYP was significantly higher (P < 0.001) than for the previous storage time. The use of this model to investigate the long-term effects of myocardial injury is described. © 1984.