In order to register pressure changes within the cauda equina, a highly sensitive pressure measuring catheter was inserted through a hole in the dural sac. Its tip was placed among the nerve roots of the cauda equina at levels L2, L3, and L4 in seven freshly removed human specimens. The cross-sectional area of the dural sac then was decreased gradually by constriction of a circular clamp. The inside diameter of the clamp and the corresponding area was determined when further reduction of the circumference of the clamp caused the first signs of a pressure increase, the critical size, among the nerve roots. The average critical size was 76.9 mm2 at the L2 level, corresponding to a diameter of 11.4 mm At the L3 level, the corresponding figures were 71.5 mm2 and 11.1 mm, and at the L4 level 64.8 mm2 and 10.6 mm, respectively. It seemed reasonable to assume that the critical sizes determined in this way could be threshold values below which a further constriction of the size of the dural sac would cause an impairment of the circulatory and/or the nerve function of the cauda equina.