The normal fine structure of the inferior pulvinar of the thalamus has been studied in Macaca mulatta. At least three types of axonal terminals can be distinguished: large, dense (type I) terminals containing rounded vesicles; medium-sized, pale (type II) terminals containing pleomorphic and flattened vesicles, and small, dense (type III) terminals containing rounded vesicles.Type I terminals form asymmetrical axo-dendritic synapses with large secondary dendrites and asymmetrical axo-axonal synapses with type II terminals, at which the latter invariably constitute the postsynaptic element. Type I terminals also form multiple, usually asymmetrical, filamentous contacts with secondary dendrites. Type II terminals form usually symmetrical axo-dendritic synaptic junctions with secondary dendrites, except in the synaptic complexes where these junctions are usually asymmetrical. Type III terminals form (asymmetrical) synaptic junctions only with dendrites; the junctions are most commonly and characteristically effected with fine, distal dendrites, and much less commonly with secondary dendrites. Synaptic contacts with somata and primary dendrites have not been found, nor ''presynaptic dendrites''. Occasional dendro-dendritic filamentous junctions have been observed. The dominant synaptological feature of the neuropil is the occurrence of localised aggregations of secondary dendrites and synaptic knobs of types I and II which have been termed synaptic complexes; the latter resemble closely the synaptic glomeruli of certain other thalamic nuclei. The synaptic complexes of the inferior pulvinar, however, are but poorly encapsulated by glial lamellae. The most meaningful subdivision of the synaptic zone of the neuropil is into proximal (i.e. secondary dendritic) and distal dendritic types, each of distinctive and fairly homogeneous synaptic organization.Two to nine days after occipital lobotomy, almost all type I terminals were found to exhibit degenerative change of the neurofilamentous hyperplastic type; no subsequent electron-dense transformation of the affected visual corticofugal profiles has been observed. Terminals of types II and III remained completely normal. A number of electron-dense profiles interpreted as somata and dendrites showing acute retrograde necrosis or indirect Wallerian degeneration, were also observed.