AbstractThe central nucleus of the inferior colliculus has been examined electron microscopically two, four and five days after section of the lateral lemniscus and four and five days after ablation of the auditory cortex. The findings confirm the differential distribution of lateral lemniscal and corticofugal fibers to the two divisions of the central nucleus and, in the case of the lateral lemniscal fibers, to the two cell types of the ventro‐lateral division.The material from brains in which the lateral lemniscus was divided contains examples of every type of terminal degeneration which has been described: swelling and glycogen deposition, swelling (often with flattening) of synaptic vesicles, neurofilamentous hyperplasia, greatly increased electron density. At two days, only the first three of these are seen; at four days, all occur together; at five days, only the electron dense type is present. In addition, at two days, a proportion of the early degenerating terminals show almost complete loss of synaptic vesicles and their replacement by the empty “shells” of complex vesicles (of Gray). Aggregations of shell fragments can be discerned at this and later survival periods clustered about presynaptic membranes left devoid of synaptic vesicles.These observations are considered in terms of recent concepts of the mode of formation and release of synaptic vesicles. It is postulated that terminals showing replacement of synaptic vesicles by shell fragments may have been excessively stimulated during or after the operation and have, thus, had a higher turn‐over of synaptic vesicles. It is suggested that, in this and other sites, accumulation of large numbers of shell fragments may be a necessary pre‐requisite for neurofilamentous hyperplasia, the protein of the shells being reorganized into neuro