The influence of transfusion upon brain water content after bleeding was studied. The variations of brain water content when transfused immediately, 60 minutes and 180 minutes after the acute arterial bleeding were studied in parts of cerebral cortex, cerebral marrow substance, brain stem and cerebellum. After transfusion, the water content was estimated by HATSCHEK's Cobaltum Chloratum method separately in free, bound and total water with the lapse of time, 10, 60 and 180 minutes. Results: 1) By transfusion immediately after bleeding, every part instantly shows a gradual increase of free and total water. It reaches to its maximum after 60 minutes and then continues or tends to slightly decrease after 180 minutes. 2) By transfusion after 60 and 180 minutes from bleeding, the water content increases markedly directly after the transfusion and then shows the same tendency as in the case of transfusing immediately after bleeding. 3) Comparing with normal ones, the largest variation of water content is found in the cerebellum. Marrow substance, brain stem and cortex follows. In comparison with the case in which only bleeding was occured and no transfusion performed, the largest difference is found in the brain stem: marrow substance, cortex and cerebellum follows. 4) The variation of water content is lowest when transfusion is treated immediately after bleeding. 5) There is almost no significant difference found between the transfusions given after 60 minutes and 180 minutes from bleeding. But comparing the variation of water content in cerebral cortex and cerebellum, the latter is slightly smaller than in the former. In cerebral marrow substance and brain stem, it shows an intensive tendency. 6) The increase of brain water content is always that of free water and no bound water is found, i. e. it means that it is a water increase of edema type.