Cerebrospinal fluid (CSF) is mainly produced by the choroid plexuses within the ventricles of the brain. The CSF circulates in a regular manner after the ventricular system and the choroids plexuses have developed, and the foramina in the fourth ventricle have opened to enable it to carry chemical information. CSF flows through the ventricular system passing over all regions of germinal activity. In this study, chick embryos were used to show the importance of CSF on neural cell proliferation in the developing cerebral cortex. The chick embryos were cannulated in situ with a fine capillary tube to drain CSF out of the ventricular system. At the same time, BrdU was administered to the embryos. After surgery the embryos were incubated for another 3 days. Quantitative measurements showed that the thicknesses of the germinal epithelium and cerebral cortex in CSF-drained embryos were less than those in the control group at the same age. The number of cells produced in the germinal epithelium of CSF-drained embryos was decreased when compared with the normal group. This study provides confirmatory evidence that CSF is important for neural cell proliferation and therefore normal development of the cerebral cortex. It is proposed that CSF is vital in controlling development of the cerebral cortex.