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N 1948, during the course of studies on seizure mechanisms, one of us (J.W.) carried out intracarotid injection of Sodium Amytal and Metrazol to investigate the mechanism of the spread of epileptic discharge between the cerebral hemispheres in man. 9-11 The intracarotid injection of Sodium Amytal was found to induce a temporary loss of function in the ipsilateral cerebral hemisphere, including aphasia when the dominant hemisphere was injected. The suggestion was made that this would be a useful technique for the determination of the lateralization of cerebral speech dominance. Approximately 80 patients were tested in this way in Japan during the period 1948-1954, using doses of 50 to 300 rag. of 10 per cent Sodium Amytal, and no complications were encountered. In the surgical treatment of focal epilepsy, the presence of speech representation has often been verified by electrical stimulation of the speech zones in the frontal and parietal opercula with the cortex exposed under local anesthesia. 4-7 Interruption of counting or naming produced by such stimulation gives positive evidence in this regard. Lack of such response, however, is not certain proof that speech is in the other hemisphere, since in some instances the electrical stimulating current does not seem to be an adequate stimulus and no effect is observed, even though speech is actually subserved by the convolutions being stimulated. In view of the obvious importance of accurate knowledge of the lateralization of speech dominance when operating near the Sy]vian regions in ambidextrous and left-handed individuals, further studies regarding the technique of intracarotid injection of Sodium Amytal seemed indicated, particularly with regard to the margin of safety in relation to dose and to the effect of accidental injection into the vertebral artery. This report concerns some experimental studies on the monkey bearing on these points, and a brief clinical report on the use of this test in a consecutive series of r patients.