The anterior and posterior roots of the third sacral nerve were used. The pieces tested varied from 20 to 50 mm. The posterior roots had a greater cross-sectional area than the corresponding anterior root. Maximum loads in kgs., regardless of cross-sectional area, was anterior root 0.2 to 2.2 and posterior root 0.5 to 3.3. The highest figure recorded being related to the greater cross-sectional area of the posterior root. The range of maximum tensile strength calculated on the cross-sectional area was, anterior root 0.4 to 2.8 kgs/sq. mm. and posterior root 0.6 to 0.3. Thus, in general the greater area the greater the maximum load. Nerve roots, like nerve trunks, are not homogeneous structures. It is not possible to evaluate the relative amounts of axon, myelin, Schwann cells, endo-neurum, loose connective tissue and blood vessels. In contrast to peripheral nerves these root nerves lack perineurim and funicular plexus formations, hence most of the cross-sectional area is made up of nerve fibers. A further factor is the caliber of the nerve fibers. Posterior root fibers were finer, on the average, than anterior roots and were stronger than anterior roots of the same dimensions. The strength of nerve roots is much less than that of peripheral nerves. This is attributed to the perineurim which is absent in the nerve roots but present in the peripheral nerves. The greatest elongation ot the elastic limit for root fibers is about 15% and complete mechanical failure occurs at approximately 25%. Each of these values is 5% less than those recorded for peripheral nerves.