The sequential changes occurring in pulmonary angiograms and xenon 133 scans during unmodified canine lung allograft rejection were compared. In angiograms, there was a minimal prolongation of the duration of the arterial phase and changes in angioarchitecture of the graft four to six days after transplantation. In lung scans, there was an accompanying decrease in ventilation and perfusion. After six days, angiographic and scanning parameters showed rapid deterioration of function in the transplanted lung. Pulmonary scanning after intravenous administration of xenon 133 should provide a convenient method for continuous evaluation of the functional changes in the transplanted lung, while angiography should be reserved for the differential diagnosis of impaired function.