Between February 1970 and January 1971 134 cases of premature infants with a birth weight of up to 2500 g were examined echoencephalographically within 48 hours of birth. The third ventricle and the brain-mantle-index (BMI) were determined. It was statistically proved that with increasing weight, advancing gestational age and growth of the head-circumference, the width of the third ventricle diminishes. The breadth of the third ventricle in premature infants and so-called “small-for-dates-infants” was found to be practically the same. An enlarged width of the third ventricle, in cases of low birth weight and low gestational age, could possibly represent a physiological phase. BMI and third ventricle correlated closely. — Conclusion: The more fully developed a premature infant, the smaller the width of the ventricular system.