In 175 Infants aged 1 day to 1 year the size of the ventricular system was measured. Standardized sections were used in coronal (approach: through the anterior fontanelle) and axial (approach: transcranial) planes. Only mature infants were taken who had no neurological signs or symptoms and who had a normal sonogram. The ventricle-hemispheric ratio (cella media width: hemispheric width) was calculated from measurements on both sides in the axial plane. Normal values are between 0.2 and 0.33. The width of the third ventricle was examined in coronal and axial sections, with mean values of 2.8 mm in the first 3 month and 3.8 mm between the 9th and 12th month of life. The fourth ventricle was examined through the anterior fontanelle and from the area of the posterior fontanelle. The latter approach resulted in the following measurements (mean values): depth 2.9 mm and 3.5 mm for the first three and the 9th to 12th month respectively. The height of the fourth ventricle was measured in the coronal plane. Mean values were 3.5 mm in the first three and 3.8 mm from the 9th to the 12th month of life.