The ventricular system is a fluid conducting system within the brain. It is filled with cerebrospinal fluid (CSF) that is largely composed of water. The ventricular system serves to bathe and cushion the brain and spinal cord within their bony confines. The ventricular system consists of two large lateral ventricles in either cerebral hemisphere (roughly eye level within the skull) that extend into the temporal lobes (temporal horns), the third and fourth ventricles, mesencephalic duct (cerebral aqueduct) and passage into the spinal cord (central canal). CSF surrounds the spinal cord and external cerebral cortex. CSF is produced by the choroid plexus within the ventricles themselves and is reabsorbed in the subarachnoid space between brain and skull. In the past it was thought that most of cerebrospinal fluid passes through arachnoid granulations into the superior sagittal venous sinus. Currently, it is thought that most of the CSF leaves the ventricular system via a path along the olfactory tracts and leaves the cranium through the cribriform plate.

Diseases of the ventricular system include abnormal enlargement (hydrocephalus) and inflammation of the CSF caused by infection or introduction of blood following trauma or hemorrhage. Interestingly, scientific study of CAT scans of the ventricles in the late 1970s revolutionized the study of mental illness. Researchers found that patients with schizophrenia had enlarged ventricles compared to healthy subjects. This became the first "evidence" that mental illness was biological in origin and led to a reinvigoration of the study of such conditions via modern scientific techniques.