The effects of Diamox on the water content in cats brain were measured. The experiments were performed by Hatschek's cobalt-chloride method taking the samples at 9 and 24 hours respectively after the injection of Diamox, in which the period of the former showed the strongest chenges and the latter showed nearly normal (the latter came back to nearly normal value) in the preliminary experiments. Concerning about the mean values of each 6 cases, the mhole water content showed 2% decrease, the free water content showed 3.4% decrease and the combined water content showed slightly decrease at 9 hours then the normal, although these water contents showed normal values at 24 hours. From these experimental results, the anticonvulsive effects of Diamox were considered to be caused by the strong dehydrating action in the brain.