AbstractThis study provides a description of the normal morphology of the inferior olive of the squirrel monkey,Saimiri sciureus, at the light and electron microscopic level. The cytoarchitecture of the inferior olive was mapped from serial transverse sections stained with cresyl violet. In common with other mammals, the inferior olive of the squirrel monkey consists of three subdivisions. The medial accessory olive includes seven subnuclei. Both the dorsal and medial accessory olives extend through approximately 90% of the total length of the inferior olivary complex. The principal olive, consisting of a dorsal and ventral lamella continuous with one another laterally, extends through the rostral 55% of the inferior olive. It is somewhat less convoluted than the principal olive of the macaque (Bowman and Sladek, '73). In most other respects, the inferior olive of the two primates is quite similar.Two patterns of dendritic arborization are noted in Golgi preparations from the caudal principal and accessory olives. Dendrites streaming away from the soma, and dendrites curling around the soma in a “ball‐like” pattern were observed in all three subdivisions of the inferior olive caudally. Simple spines are occasionally seen on the soma, and a few simple or club‐shaped spines were noted on the proximal portion of the dendritic arborization. Spines are more numerous on distal portions of the dendritic tree, however, and include simple, filiform, club‐shaped and occasionally complex, or racemous, spiny appendages.Viewed in the electron microscope, most inferior olivary neurons are seen to contain the typical organelles with the usual conformation and distribution. Rarely, a neuron with an indented nucleus and a thin rim of cytoplasm containing a paucity of organelles and a wispy endoplasmic reticulum is encountered. Axon terminals containing either clear round or clear pleomorphic vesicles are seen in all three olivary subdivisions. In a random survey of 706 axon terminals, 54% contained predominantly clear round vesicles. Large dense cored vesicles are seen in varying numbers in both types of terminals. Rarely, profiles containing mainly large dense cored vesicles are observed.Axosomatic synapses involving both types of clear vesicle containing terminals are occasionally encountered. Such synapses are symmetrical, regardless of the type of vesicle found in the axon terminal. Axodendritic synapses involving round vesicle containing terminals are asymmetrical, while those involving pleomorphic vesicle containing terminals are usually, but not invariably, symmetrical. Axodendritic synapses occur at all levels of the dendritic tree. Very rarely, synapses between an axon terminal and a profile resembling a dendrite, but containing pleomorphic vesicles, has been observed.Synaptic clusters, consisting of a central core of small dendritic elements surrounded by both round and pleomorphic vesicle containing terminals, are found in all three subdivisions of the inferior olive. The dendritic elements, which resemblespines, contain a flocculent cytoplasm, mitochondria, multivesicular bodies and smooth endoplasmic reticulum. Evidence from some electronmicrographs suggests that the dendritic core is composed of racemous processes. Gap junctions are occasionally encountered coupling adjacent dendrites within the core.Gap junctions are encountered in all three subdivisions of the inferior olive, but are commonest in the two accessory olives. While most gap junctions couple den‐drites, one example of an axosomatic gap junction has been