Pathologic and bacteriologic studies were carried out in all 24 dogs subjected to arteriovenous anastomoses and bacteremia. The single etiologic agent, Streptococcus mitis , was stained and cultured from 10 of the 11 hearts with active endocarditis (one animal had a healed tricuspid valvulitis) and from 16 of 17 animals with patent fistulas and active caval vegetations. The latter lesions occurred shortly after the initiation of injections and served as an important source of bacteria to the blood stream and heart valves. The endocarditis resembled that found in human cases with destructive and reparative phenomena coexisting. Focal myocarditic lesions and cardiac failure were noted only in animals with endocarditis. The other secondary (embolic) lesions were similar to those seen in man with the exception that in the dog the lesions had a suppurative tendency. Acute diffuse proliferative glomerulonephritis was found in animals with endocarditis and in those with infected fistulas and bacteremia.