One-dimensional echoencephalographic investigations were made of the 3rd ventricle with the Siemens-Krautkrämer Unit USM 1 in 1841 subjects. 200 premature infants with a birth weight of up to 2500 premature infants with a birth weight of up to 2500 g were examined within 48 hours of birth. 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 material of 1641 children and adults shows the following results: The first phase up to 21/2 years is characterised by a rapid increase in size. The second phase covers the age group up to about the 16th to 20th year and ist characterised by a kind of plateau without significant change in the width of the 3rd ventricle. In the third phase concerning ages from 16 to 20 up to 57 to 58 years, a slow increase to higher values can be observed. The fourth phase, from the 57/58th to 78/79th years, shows a distinctly higher increase than the third phase. Results obtained beyond the 78/79th year were not uniform but rather heterogenous and unpredictable, questioning the concept of a further "physiological" development of the 3rd ventricle. A linear relationship between age and the 3rd ventricle can be demonstrated up to about the end of the 8th decade. Normal values for the 3rd ventricle should not be given in future, without indicating the age.