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Internal jugular blood sampled by percutaneous puncture below the mastoid process has been studied extensively in man on the assumption that it represents cerebral venous blood without significant admixture of extracerebral venous blood. A series of 67 internal jugular blood studies revealed 5 markedly deviant observations characterized by a very slow rate of inert gas saturation (i.e. a very slow blood perfusion) and a narrow arteriovenous oxygen difference. Similar findings were obtained in two studies of external jugular blood. These observations and technical details regarding the venous punctures suggest that the five deviant cases were grossly contaminated by extracerebral venous blood, the venous blood coming from some other vein in the neck than the superior bulb of the internal jugular vein. Although such cases are relatively rare, they deviate so markedly from essentially uncontaminated cases that their exclusion adds considerably to the validity of internal jugular blood for the study of cerebral blood flow and metabolism in man. Submitted on March 2, 1960