This study of the effects of certain drugs, gas mixtures, and other stimuli on the cerebrospinal fluid pressure corroborates previous conclusions that alterations in cerebral blood flow are reflected by similar changes in the cerebrospinal fluid pressure in the same direction. The hypothesis is advanced that the elevations and depressions in cerebrospinal fluid pressure resulting from these agents are due to the respective increased and decreased volume of the vascular structures of the brain, expanding within its rigid container.