Enamel and superficial layers of dentine of the lateral surface of the incisors of anesthetized white rats are ground away with a size 1/2 dental burr. ,Care is taken to leave a thin (30[mu]) layer of dentine over the delicate pulp tissue, and mineral oil is used as a lubricant and to increase transparency of dentine in final stages of operation. Individual erythrocytes in capillaries, movement of macrophages through pulp, and other minute details are visible in this prepn. The window is pushed beyond the living pulp in 3-4 days by the growth of the tooth. Observations through such windows indicate that retracting stress on incisors retards or stops blood flow through vessels of pulp as does stimulation of the cervical sympathetic chain. Venous flow appears to stop before arterial flow, and no change in arteriolar caliber is detectable, indicating the possibility of a special mechanism for control of blood circulation in tooth pulp. Attempts to apply similar methods to cats, dogs, and monkeys have been unsuccessful.