In an attempt to examine the morphologic changes which take place in the spinal dorsal horn as a consequence of peripheral nerve injury, the superficial radial nerve was cut and prevented from regenerating in adult cats. In laminae I-V of the 7th cervical segment of spinal dorsal horn at 2 weeks, 1 and 2 months following transection, small and medium-sized dendritic shafts developed membrane-bounded cavities. These cavities varied in number and size in each dendrite and were sometimes connected to the agranular reticulum. Large cavities often hollowed out a dendrite, leaving little remaining cytoplasm between the cavity membrane and the plasma membrane. Small and large cavities were frequently found open to the intercellular space. Synaptic glomeruli showed a loss of small dendrites with empty scalloped depressions in the central axonal endings of these structures left exposed to enlarged intercellular spaces. In addition, clusters of many enlarged oval or irregular intercellular spaces were found in the neuropil where many cavitated dendrites were observed. At least some of these intercellular spaces were thought to be derived from the loss of dendrites. From these observations we conclude that small and medium-sized dendrites involute through cavitation and eventually disappear from the spinal dorsal horn when primary input is disturbed by transection of peripheral sensory nerves.