When emboli of rabbit blood measuring 0.5 min. in average diameter were injected into the ear vein of healthy adult male rabbits, they either became adherent to the walls of the pulmonary arteries or impacted in them. In either case an acute arteritis accompanied by endothelial proliferation regularly occurred. This process was reversible in some instances but in others was followed by organization of emboli which resulted in eccentric fibroelastic thickening of the intima or formation of fibrous intravascular bridges. Fatty degeneration, calcification, hemorrhage, and atheroma were absent. The lesions were self-limited and healed. Theyrwere not progressive.