The cerebrospinal fluid (CSF) composition was studied in 54 premature infants. The pregnancy was normal and the delivery normal and non traumatic in all of them, and the 5 minutes Apgar score ranged from 6 to 9. No abnormalities were found on physical examination including neurological examination. Blood cell countings and blood gasometry were normal. CSF composition was studied as to: total cell count and total protein, glucose, bilirrubin and hemoglobin concentrations. Data found permit to stablish as physiologic the following values: leucocytes, until 16 per cumm; erithrocytes, until 1,280 per cumm; total protein content until 300 mgm/100 ml; bilirrubin until 80 µM/l; hemoglobin until 8 µM/1; glucose, two thirds of the concentration found in the blood. Protein, bilirrubin and hemoglobin are significantly increased as compared to values found for the CSF of 79 fullterm normal newborn babies evaluated previously. Hemoglobin was not detected in the CSF of any full term newborn baby. The differences found are probably due to a less efficient blood-CSF barrier in premature infants as compared to full-term newborn babies.