Recent physiological evidence shows that the small, thinly myelinated fibers of peripheral nerves are concerned with the conduction of pain (Heinbecker, Bishop and O'Leary). Evidence for the participation of fiber elements in the tract of Lissauer of the spinal cord in the transmission of pain seems adequate (Ranson and Billingsley). It is necessary, therefore, to determine how large is the number of small, thinly myelinated fibers which enter the tract of Lissauer from the dorsal roots. To examine this tract, the seventh lumbar segment of the spinal cord of the cat with attached dorsal roots was isolated and divided into lateral halves. One half was fixed in ammoniated alcohol for the pyridine silver technique, the other in 2% osmic acid by immersion. The osmic acid cross-sections revealed that Lissauer's tract contains closely packed, small, thinly myelinated fibers. The silver pyridine preparations showed from 1 1/2 to 2 times more fine axons than could be accounted for by the number of fibers apparent in the osmic acid sections. The additional axons were those of non-myelinated fibers. Evidence secured by Ranson through degeneration of the dorsal roots indicates that in the lumbosacral cord (cat) the tract has medial and lateral divisions; the lateral is primarily endogenous to the cord, the medial contains fibers which have entered from the dorsal roots. Accordingly, in 7 cats, under ether anesthesia, the dorsal roots (levels L. 5 through S. 3) were ligated and cut (3) or crushed (4) between the spinal ganglia and the cord. The cats were killed after 20 to 100 days and the material prepared by the silver pyridine, 2% osmic acid and Marchi methods. The results show that a majority of the small, thinly myelinated fibers in the medial half of the tract of Lissauer degenerate (cord levels L. 6 and L. 7).