A hypothesis of the existence of a negative pressure in the perivascular space around the brachial plexus as the one that exists in the peridural space is proposed. To prove this hypothesis, a special designed manometer adapted to the needle for performing the punction of the fascia using Winnie's technique was used. In ten cases the existence of a subatmospheric pressure was proved, in the perivascular space. The measurement of this negtive pressure suggests a proportion between the values found and the ages of the patients. The highest pressure was registered in the youngest patients.293324329