The numbers of large and intermediate diameter myelinated fibers of 17 ventral lumbar (L3, L4 and L5) spinal roots (vsr) of man (ages 17-81 years) have been evaluated using the improved methods of histologic processing and morphometry now available, semi-automatic methods of measurement and programmed calculation and plotting. A population of small diameter myelinated fibers as found in thoracic (T11) vsr, which are presumed to be preganglionic sympathetic fibers, were essentially absent from L3, L4 and L5 roots. On the average, there were 5716 (70%) large diameter fibers and 2625 (30%) intermediate diameter fibers in the L3 ventral spinal root. Comparable figures for L4 vsr were 4900 (70%) and 2152 30%) and for L5 vsr were 5043 (75%) and 1559 (25%). On the assumption that these lumbar roots contain few if any small diameter myelinated pre-ganglionic sympathetic axons, the large diameter fibers correspond to alpha motoneuron axons and the small diameter fibers to gamma motoneuron axons. With age, a decrease in the number of myelinated fibers was estimated to be 350 fibers per decade (approximately 5% per decade).