In order to determine the enzyme activity, as expressed in Reitman-Frankel unit, of GOT isozyme present in whole homogenate, mitochondrial fraction and supernatant fraction were prepared from brain tissues of normal adult rabbit, by a differential centrifugal method. The effect of thiopental on the GOT isozyme activity in each fraction was determined and the following results were obtained. 1) The activity of GOT isozyme in whole homogenate of normal rabbit brain tissues was found to be 545±2.608 units/mg of wet weight whereas the corresponding figure for the supernatant GOT isozyme was 512±3.081 and the value for the mitochondrial GOT isozyme was found to be 34.9±1.224. 2) The supernatant GOT isozyme existing in a floating status within the cytoplasm accounted for 94 percent followed by 6.35 percent of mitochondrial GOT isozyme. 3) The activated-peak of mitochondrial GOT isozyme contained in the whole homogenate of adult rabbit brain tissues was found to be at #15 on the tube of elution in comparison to that of #73 for supernatant GOT isozyme, as analyzed by the DEAE-Cellulose column chromasography. 4) The supernant GOT isozyme from the thiopentaltreated brain was proportionaly distorted while mitochondrial GOT isozyme was not influenced. Fro example, treated with thiopental, the supernatant GOT isozyme was divided to be #63 & #73 on the tube in comparison to #15 for the mitochondrial GOT isozyme. 5) The activity of supernatant isozyme was proportionaly reduced as the concentration of thiopental. 6) Fifty percent inhibition dose(1se) of thiopental on the supernatant GOT isozyme was found to be 0.63mM. 7) The inhibitory effect of thiopental on the supernatant GOT isozyme was very high significantly by the statistics. 8) The mchanism by which thiopental inhibits the supernatant GOT isozyme in the adult rabbit brain was found to bh uncompetitive inhibition as its Michaelis-Menten constant Km=58.07mM demonstrated. In view of the above finding it is suggested that the thiopental inhibited selectively the activity of supernatant GOT isozyme of the adult rabbit brain tissues while it did not inhibitnificantly by the statistics. 8) The mechanism by which thiopental inhibits the supernatant GOT isozyme in the adult rabbit brain was found to bh uncompetitive inhibition as it Michaelis-Menten constant of Km=58.07 mM demonstrated. In view of the above findings it is suggested that the thiopental inhibited selectively the activity of supernatant GOT isozyme of the adult rabbit brain tissues while it did not inhibit that of mitochondrial GOT isozyme. The GOT isozyme of adult rabbit brain tissues was divided into thiopental-sensitive GOT isozyme(supernatant GOT isozyme) and thiopental insensitive GOT isozyme(mitochondrial GOT isozyme) Furthermore, it is suggested that the cellular function of the brain can be somewhat hindered, when thiopental is injected into the brain cell, while mitosis of the brain cell is not influenced.