The importance of efficient ventilation of dairy barns has been emphasized by the recent investigation of the causes conducive to the development and contagion of tuberculosis in cattle. Bulletin No. 7, of United States Bureau of Animal Industry, on Investigations Concerning Bovine Tuberculosis, says: “Fully nine-tenths of all diseased animals have been infected by inhaling the tubercle bacilli dried and suspended in the air.” Without proper ventilation the tubercle bacilli and other disease germs must be much more readily inhaled, and it is a matter of record that the worst diseased herds are found in poorly lighted and poorly ventilated buildings; while range cattle and other stock, seldom sheltered or confined in close buildings, shows practical immunity from the disease. Most barns have no system of ventilation except the introduction of outside air through doors and windows, and farm buildings generally "have an inadequate supply of these. In view of the importance of this subject, the plan of ventilation, and incidentally some other features, of an experimental barn recently constructed at the Iowa Agricultural College is herein presented.