Convulsive seizures were caused by metrazol injection in cats, and the water content of the brain was measured at the cerebral gray matter, white matter, brain stem and cerebellum by the cobalt-chloride method. The water content became a little more at the preconvulsive stadium and it became the most at the acme as well as the postconvulsive stadium. Then it showed a gradual decrease to the normal as time elapsed. The preconvulsive increase of the water content could be considered to cause the convulsion, and the convulsion could also be considered to cause the increase of the water content, because of the maximal increase at the acme and postconvulsive stadium, which suggested some relation between the water content and the activity of the central nervous system.