Scientists have speculated as to the nature of cerebrospinal fluid (CSF) - "the third circulation" or "vital spirit" for centuries. Just what is this mysterious bathing solution of the central nervous system? Is it a vehicle of transport for an "animal spirit" directing all our activities, as thought by Galen (Singer, 1956) or but a "modified tap water" (Halliburton, 1917)? With the advent of lumbar puncture, cerebrospinal fluid has become a readily available and important means of studying disease affecting the nervous system. In recent years, many sophisticated tools including computer guided gas chromatography/mass spectrometry and high performance liquid chromatography have made it possible to identify and quantify many constituents of this fluid. The CSF has an enormous, though largely "untapped", potential in aiding diagnosis and evaluating treatment of many neurological, psychiatric and systemic disorders. As the ependyma is only a diffusional barrier between the CSF and extracellular fluid of the brain for many compounds, changes in the concentration of these compounds in the CSF may reflect disease processes in the brain.