The morphological features of the rejection of pulmonary homografts in the dog are described based on 44 consecutive autografts and homografts completed with an operative mortality of less than 5%. The primary morphological features in the homografts consist of 1) prominent perivascular round cell infiltrates beginning at about 4 to 5 days with the peak at 7 to 8 days, 2) coincident intra-alveolar edema and round cell infiltrate, and 3) terminal massive necrosis and hemorrhage of the entire homograft without evidence of thrombosis at 8 to 11 days. These features were not observed in the autografts. Functional studies utilizing peripheral arterial pO2 determinations indicated that functional impairment correlated with the anatomic alterations.