A series of experiments in calves designed to correlate functional changes in pulmonary blood flow with the histology of rejection of lung allografts was performed. The changes in blood flow to the allograft were detected by a scintiscan technique and studied in the presence of progressive impairment of blood flow to the contralateral lung. The results of these experiments indicate that the histology of rejection in the calf varies in degree but not kind from that in the dog and the baboon. Histologic changes were first seen in the alveoli and small vessels and bronchioles. These morphologic lesions of rejection were associated with a progressive decline in blood flow to the allograft which began within one to four days and resulted in almost complete cessation of blood flow to the allograft within ten days.