The response of the cerebrospinal fluid pH to changes in the blood is of considerable physiological and clinical interest. The role of the cerebrospinal fluid in the control of pulmonary ventilation is pointed out. The main aspects of the neurologic manifestations and the cerebrospinal fluid changes in patients with hydrogen ion disturbances are discussed. The purpose of this paper is the study of the chloride/total CO2 ratio in the cerebrospinal fluid. In 37 normal individuals the ratio ranged from 4.8 to 5.8. In 11 patients with respiratory acidosis it ranged between 2.0 and 4.7. In 10 patients with metabolic acidosis due to severe diabetic coma the ratio ranged between 6.5 and 22. It is emphasized the importance of this simple test in the cerebrospinal fluid of neurologic patients with hydrogen ion disturbances.