The presence of a case of heart block in the medical clinic usually evokes more than ordinary interest. A heart with a ventricle beating at a rate that may be only one-half or one-third its normal rate and often efficiently meeting the demands of the body is but one of the many examples of the provision nature has made to care for contingencies. That a patient with heart block may successfully pass through a disease which is frequently fatal is worthy of comment. A patient suffering from uremia may develop heart block, and uremia may develop in one who has permanent heart block. Heart block may be temporary or permanent. The poisons of rheumatic fever, chorea, tonsillitis, diphtheria, pneumonia, syphilis, scarlet fever and septic infections may inaugurate heart block. The toxemia of chronic infections and metabolic disturbances, such as chronic interstitial nephritis and diabetes mellitus, may be an etiologic factor.