The recent studies on the extracellular fluid in cancer patients have demonstrated a reduced blood volume and an increased interstitial fluid volume. By measurements of the extracellular fluid in patients with gastric cancer and gastric ulcer. I could compare them with the fluid in healthy persons and the following results were obtained: 1. On the extracellular fluid, each of cancer and ulcer patients has an increased volume more than that of healthy persons and the remarkable increase was observed in especially the former and the patients in such poor condition that lost the body weight as much as 12kg. or more than that, but the influence of passage disturbance of the pylorus was not recognized npon the extracellular fluid of gastric cancer. 2. The circulatory plasma volume was also recognized its increase more than the volume of healthy adults. The parallel relation between the plasma volume and the extracellular fluid was not confirmed on the whole except a few cases. 3. On the circulatory blood volume, there was no special difference among them. 4. The cell mass was reduced more in cancer and ulcer patients either than in healthy persons and in cancer patients the reduction was remarkable. 5. The alteration on the extracellular fluid which caused by blood transfusion was very little in a short term. The character of extracellular fluid is very complicated and is very difficult to explain about it, but I can assume that the remarkable increase in cancer patients is caused by the spread of the thiocyanate space.