THIS report concerns a method proposed for the roentgenologic visualization of protruded intervertebral disks. All of the usual procedures employed for this purpose have disadvantages. The positive contrast media, of which lipiodol is the one most commonly used, may prove locally irritating and necessitate the opening of the dura at the time of operation. Thorium dioxide is a potentially radioactive substance and its use has not been generally accepted. The popular air myelogram or spinogram, which is a film taken of air in the subarachnoid space, does not compare with other methods in diagnostic accuracy. It has the added disadvantage that unless the patient remains supine for at least twenty-four hours the air will enter the cranial cavity, causing intense headache. The use of oxygen instead of air, as recommended by Poppen (10), will evidently shorten the interval that the patient has to stay in bed. In our attempt to find a more suitable method of demonstrating protruded intervertebral disks, and more particularly to avoid the headache and discomfort caused by air myelography, it was decided to introduce the air into the epidural rather than the subarachnoid space. It was recognized that Odom (8, 9) and others (1, 2, 4, 5, 7) had utilized this space for the giving of anesthetics, but no reference was found in the literature regarding the injection into it of a contrast medium for clinical roentgenographic purposes. The spinal epidural space is that area within the spinal canal which surrounds the dura, extending the entire length of the spinal canal, from the rim of the foramen magnum to the coccyx. Its upper limits are tightly sealed at the foramen magnum so that air or other substances injected into the space cannot enter the cranial cavity. Nor is there any communication between this space and the subdural or subarachnoid spaces. It is larger than is commonly realized; in the lumbar area its total diameter exceeds that of the subarachnoid space (Fig. 1). Heldt and Moloney (6), as well as Soresi (11), noted a negative pressure within the epidural space. Whether this is a true negative pressure or merely a spurious one caused by indentation of the dura by the point of the needle has been debated, recently by Eaton (3). Without detailing at this time the reasons for our conviction, it is our opinion that in certain patients under certain conditions, principally those having to do with position, a true negative pressure exists. Whether this be true or spurious, it can be utilized in locating the space, by attaching to the needle a manometer which registers negative pressures, or, more simply, by placing a drop of fluid in the open end of the needle, which is inserted slowly until the drop is aspirated. This technic may be employed in doing epidural punctures, but at times, in spite of the greatest caution, the subarachnoid space will inadvertently be entered before an indication of negative pressure is obtained.