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A thorough investigation of 10 healthy males, 26 to 46 years old, has been performed. One and a half ml. of their own erythrocytes, labeled with P 32 , was injected intravenously and dilution curves obtained simultaneously from one carotid artery and the two jugular bulbs. Thereafter, the same amount of labeled blood was injected into both carotid arteries, following one another, and dilution curves from the two jugular bulbs were simultaneously obtained. A total of seven dilution curves was collected in each case. The percutaneous punctures of the carotids were high enough in the neck so that it is highly probable that the injections were made into the internal carotid arteries. Calculations of the cerebral blood flow and the cerebral blood volume in absolute values were performed. The mean value of the cerebral blood flow amounted to 876 ml/min., i.e., 11.4 per cent of the cardiac output. The pool of the brain was calculated to about 100 ml. When a carotid artery was injected, the appearance and peak concentration times estimated from the dilution curves obtained from the ipsilateral bulbs were the same whether the injection was performed in the right or left side. The corresponding times for the dilution curves obtained from the contralateral bulb agreed, whichever the side on which the injection was performned. They were, however, one second longer than the ipsilateral ones. The determination of circulation time is of diagnostic value, according to an earlier publication. A simplified bloodless method has also been tried.