The sciatic nerve of newborn rats (less than or equal to 16 h old) was crushed with a watchmaker forceps. During the first 4 weeks after the injury, examination of ipsilateral L4 through L6 dorsal root ganglia, their dorsal nerve roots, and the dorsal funiculus revealed the presence of degenerating myelin and axons. Chromatolysis was not observed. In the spinal cord, the degenerating argyrophilia was restricted to the medial part of the dorsal funiculus (fasciculus gracilis). This is interpreted as transganglionic degeneration of the central processes of the pseudounipolar cells. Twelve weeks after nerve crush, there was a noticeable reduction in the size of the leg, foot, and muscles innervated by the sciatic nerve as well as a substantial loss (P less than 0.001) of neurons and myelinated axons in ipsilateral spinal ganglia and their dorsal nerve roots. The reduction was most prominent among the larger sensory neurons (greater than 40 microns) and the larger myelinated axons. A total loss of about 60% of sensory neurons was found in the L4 through L6 spinal ganglia. About 58 and 64% of the myelinated axons were lost in L4 and L5 dorsal roots, respectively. The remaining perikarya and dorsal root axons were hypoplastic.