The effects produced by a great variety of drugs on the cerebrospinal fluid pressure have been studied by a number of investigators. The substances used include such drugs as ether, chloroform, strychnin, atropin, amylnitrite and pilocarpin, and a number of organ extracts, such as epinephrin, pituitary extract, choroid plexus, and brain substances. More recently, since the work of Weed and McKibben, the effect of hypotonic solutions on raising the pressure has been frequently studied. There is considerable difference of opinion, as expressed in the literature, concerning the effect of many of these substances. Often where there is an agreement on the fact that certain substances will increase the cerebrospinal fluid pressure, there is no entire agreement as to how this is accomplished, some believing that it is simply the result of the effect of the pressor action on the circulation being transmitted to the fluid pressure. The present discussion concerns