The total protein content of the CSF of 1,011 cases was studied in order to explore the influence of the age and of the sex of the patients. The CSF samples studied were collected from the cisterna magna. They did not show changes regarding to pressure, aspect, color and cytology. Globulin tests were negative (Pandy and Takata-Ara) as well as complement fixation tests for syphilis and cysticercosis. The protein content was determined by the trichloracetic acid method as proposed by Bossak and coworkers. The normal limits for physiologic changes were those determined by the study of another series of cases. These data were previously reported. The protein contents of the CSF samples studied in this publication are between such limits. The influence of the age was studied between the limits of 3 months and 70 years. It was found that the mean values tend to decrease from the first group (patients 3-6 months old) until one year of age. From one to 40 years the CSF protein content shows no changes which may be related to the age. Above 40 years the protein content of the CSF tends to increase. This tendency is more marked among the elder patients. The influence of the sex was explored among people whose age varied from 16 to 50 years (596 of all the patients). The mean concentration of the CSF proteins found for men was higher than that found for women. This difference is statistically significant. These data confirm the results registered by other authors. They permit to define such physiologic changes in respect to the method employed for the determination of the CSF protein content.