Is mercury injected into the body excreted into the spinal fluid? This question occupied us with practical and theoretical points of view. On the practical side, we were interested in knowing how much we can count on the circulation of mercury in the spinal fluid and, therefore, on its direct action on the nervous tissue due to the communication of the perivascular (and pericellular) spaces with the sub-arachnoid. If mercury is released into the spinal fluid, it is necessary to search for the therapeutic effect (syphilis of the nervous system) of the drug that quickly and in large quantities passes into the spinal fluid. On the theoretical side, the issue of mercury release is of interest for solving the broader issue of the nature of spinal fluid in general. As is known, there is currently no agreement on this account. Is the spinal fluid transudate, the secretion of the vascular plexus epithelium or the sui generis lymph of the brain itself. In favor of the second1 views are inclined by Schultze, Imamura, Raubitschek, Molt, and others in favor of the last but Spina2 (also Lewandovsky and Blumenthal3. The first view is generally accepted. We thought that the saturation of blood with mercury, which happens with prolonged introduction of it into the body, should lead to the appearance of at least traces of it in the spinal fluid, if the latter is transudate. If the last secret, then apriori nothing can be predicted; extraction depends on the chemical and physical properties of the epithelium itself; the epithelium can secerne one substance and not pass another. The number of substances found so far in the spinal fluid when injected into the body is very limited. When the brain (and membranes) was normal, the substances introduced by the authors did not completely enter the spinal fluid. Widal, Monod4, Sicard was found in tuberculous meningitis iod when giving it during 2-3 days for 3-5 grams only in 3 cases. Guinon and Simon found only 1/2 cases of tuberculous meningitis; no iodine was found in cases of cerebrospinal meningitis. With uremia, Costaigne found iod and methylene blue. Sicard and Widal didnt find it. Gilbert and Castaigne found bile pigment in jaundice. Sicard denies. Archard Loeper5 did not find the lithium when it was injected into the blood. Regarding the fate of mercury introduced into the organism, there are no indications in the literature6.