Corpora amylacea (brain sand) were, hypothetically, considered as precipitates of a substance present in the cerebrospinal fluid (CSF). Melatonin, a non-polar indol, present in CSF could enter fibrous astrocytes and could generate corpora. The site of secretion of the corpora generating substance and the flow of CSF should dictate the distribution pattern of corpora. Neuropeptides present in the fluid were also considered as a possible precipitating agents. The distribution of corpora and the action of intrathecal vincristine suggested that CSF is recirculated through the ventricular system. A complete cycle of ventricular fluid could set the pace of the 100 minute ultradian rhythm found in human beings. Arachnoid granulations were not in an area of high volume of flow of the fluid and probably do not act as a major site of fluid reabsorption.