Twenty-one patients were anesthetized by intravenous injection of thiamylal sodium and changes in blood and interstitial fluid were observed before and after anesthesia. 1) The specific gravity of blood and plasma, plasma protein and hematcrit slightly increased after intravenous administration of thimylal sodium, while the water content of blood and pleasma decreased. 2) Among the plasma electrolytes, plasma nartrium and chloride decreased, while plasma pottasium increased. 3) A decrease of plasma volume and an increase of interstitial fluid were observed. 4) The changes of factors mentioned above is regarded to be caused not only by blood concentrasion but also by transmission of plasma fluid to extracellular fluid.