In order to follow up the effect of _isolated cooling of the neck and the back of the head_ on consciousness, body temperature, and circulation, this was undertaken in three special experiments. The experimental subject lay horizontal; the back of the head and the neck were dipped into a receptacle through which water of corresponding temperature was continuously run. In an experiment of 3 hours duration there occurred small temperature drops of not more than 0.8° C. [1.4° F.]. The water temperature was 1° to 2° C. [34° to 35° F.]. In one case after 50 minutes a marked sleepiness occurred which changed over into a deep narcosis. The heart activity was variable, and obvious bradycardia could not be observed. Irregularity never developed. Changes were not seen in the electrocardiograph. On the other hand in all three subjects the spinal fluid pressure was markedly increased after the ending of the experiment to maximal values of 300 mm. After the experiment, ataxia and definite Romberg phenomena were observed, as well as exaggeration of the normal reflexes; pathological reflexes were absent. _IV. Blood, Spinal Fluid, and Urine During Freezing_ The _differential blood smears_ showed no special features during cooling. On the other hand the number of white and red blood corpuscles shows a regular change. The _number of leukocytes_ rapidly increases, roughly with the beginning of the steeper temperature drop at about 35° C. [95° F.] rectal temperature to values of from 25,000 to 27,000 per cu. mm. After one hour a maximum may be reached and a falling-off begins in the number of leukocytes, while the body temperature falls still further. The number of red corpuscles undergoes an increase, though to a relatively small degree, which in its course resembles the change in the _number of leukocytes_. We saw increases up to 20 percent. This increase is interrupted even earlier than the increase in the number of leukocytes, so that both curves give no reflection of the temperature curve. The increase of the erythrocytes corresponded to the increase of the hemoglobin of from 10 to 20 percent. A reduction of the fragility of the red corpuscles could not be demonstrated with certainty, on the other hand, although in three experiments a definite hemolysis occurred. The viscosity regularly increases with the beginning of the fall in temperature. The rise can reach values up to 7.8. This rise occurs very early, indeed, already at body temperatures of 35° C. [95° F.]. After that the values remain relatively constant with further temperature falls. The _albumen content of the plasma_ was likewise increased after the experiment, on the average by 1 percent of the absolute value. Since these measurements could not be made as often as those of viscosity for technical reasons, the connection with the progress of the viscosity remained unclear. Such a connection could not be recognized from the absolute values obtained.