The study was made in order to verify if the bilirubin content of the cerebrospinal fluid (CSF) is constant during the first week of life, and their relations to: the concentrations of total proteins in the CSF and bilirubin in the blood serum; the red blood cells and leucocytes present in the CSF sample. The study was made in 79 normal newborns without perinatal problems or obstetrical abnormalities. The gestational ages were stablished through maternal anamnesis and the clinical evaluation of the newborn was made according to the patterns established by Usher & col., Lubchenko & col. and Lubchenko. All the pregnancies were over 37 weeks. The vital conditions of the newborn immediately after the delivery were calculated according to the patterns proposed by Apgar and Apgar & James. The first complete clinical examination was made in the first twelve hours of life. The cranial sizes were situated within the limits accepted as normal. The deliveries were head-first (only two were breech presentation). The cry was immediate after birth, the breathing well established in the first minute of life and the cut of the umbelical cord was made after the first respiratory movement. The deliveries were made in delivery rooms or surgical rooms always assisted by an obstetrician and a pediatrician. The collecting of venous blood to determine the serum concentration of bilirubin was made soon after lumbar punction for collecting CSF. Samples were collected in the second day of life in all the 79 newborns studied; in 40 of them, new samples were collected in the 7th day of life. CSF samples were analyzed as to: total white cells count; total red blood cells count; total protein concentration; bilirubin and haemoglobin concentrations according to the Kronholm & Lintrup nomogram. General data pertinent to weight, height and cranial size of the newborn children studied were the following: weight: 3.213.1 ± 286.9 g; height: 49.24 ± 1.12 cm; cranial size: 35.29 ± 0.89 cm. Most CSF supernadant were either xanthochromic or slightly xanthochromic. The following values were found for cells, proteins and pigments in the CSF: 1. Total white cell count at the second day of life: mean of 4.44 cells/cu.mm., standard deviation 3.83; in the seventh day the average of cells was 4.40 cu.mm. and standard deviation 3.33; no significant difference was found between these two values. 2. Red blood cell count in the second day of life: mean 155.76 red blood cells/cu.mm. with standard deviation of 223.48; in the seventh day the mean was 74.10 red blood cells/cu.mm. and standard deviation was 123.72. The average number of CSF red blood cells at the second day of life was found to be larger than the mean value at the seventh day. 3. Total protein concentration: the average of total protein (mgm/100 ml) was 71.44 with standard deviation of 24.15 in the second day of life; at the seventh day the average was 69.27 with standard deviation of 22.21. There were no significant statistical differences between these values. 4. No haemoglobin was found but bilirubin was always present. The average of bilirubin at the second day of life was 1.96 micromol/liter with standard deviation of 0.17; in the seventh day of life the average was 1.89 micromol/liter with standard deviation of 0.20. The difference between the second and seventh days of life, although statistically significant, is inconspicuous for clinical and diagnostic purposes. No significant correlation was found between the bilirubin concentration in the CSF, and the respective white cells, erythrocytes or protein concentration. No significant correlation was found between protein and bilirubin in normal newborn's CSF. No significant correlation was found between bilirubin concentration in the serum and that of the CSF.