The mechanisms leading to the hyperacute rejection of a vascularized xenograft are still incompletely understood. The first stage of the rejection process is when blood of the recipient comes into contact with the endothelium of the xenograft. A working heart model was used to examine endothelium-related processes and their impact on organ function. Pig hearts were perfused with porcine (autologous) or human (xenogeneic) blood. Cardiac function was evaluated by calculating the stroke work index, arteriovenous oxygen, coronary flow, and resistance. PgF1a as a marker of endothelial activation, its antagonist TXB2, and myoglobin reflecting myocardial damage were measured in the hemoperfusate. H&E and PAS staining and immunohistological demonstration of factor VIII-related antigen was performed. Xenogeneic perfused porcine hearts showed significantly less stroke work, a higher arteriovenous oxygen difference, and an increased coronary resistance. Factor VIII-related antigen could not be demonstrated immunohistologically on the endothelium after xenogeneic perfusion. PgF1a levels were significantly higher in the xenogeneic hemoperfusate, indicating endothelial cell activation. The concentration of myoglobin in the hemoperfusate remained within normal values and was similar during autologous and xenogeneic perfusion. Therefore endothelium-related processes are likely to affect the coronary circulation—thus being one mechanism leading to diminished cardiac performance during hyperacute rejection.