In this disertation, we have tested a new hypothesis about CSF dynamics, according to which CSF pressure does not depend on CSF secretion, unidirectional circulation and absorption, but it is a consequence of biophysical characteristics of cranial and spinal space, according to the law of fluid mechanics. CSF pressure and volume changes, observed in different „body“ positions of the new CSF system model, which consists of the cranial part with unchangeable volume, and the spinal part with a volume that can be changed, were compared with the observations obtained from anaesthetized cats and rabbits in control conditions, as well as after producing cervical stenosis and aqueduct of Sylvius blockade. In an upright position, in control animals an appearance of subatmospheric pressure inside the cranium occures, which is in accordance with the law of fluid mechanics, and in opposition with the generally accepted hypothesis about CSF dynamics. This implies that a much better brain perfusion exists in an upright body position than it was previously believed (an important evolutionary adjustment). A performed analysis of hydrostatic pressure gradients inside the craniospinal CSF space in all of the tested body positions indicates that CSF circulation in an unidirectional manner from the ventricles into the cortical and spinal subarachnoidal space is impossible. In the case of impaired craniospinal communication (cervical stenosis, aqueductal blockade), the pressure inside the ventricles becomes more positive. The observed results are in accordance with the new microcirculatory hypothesis of CSF physiology, according to which CSF pressure and volume are regulated by osmotic and hydrostatic forces at the brain and spinal cord capillary level.