The transentorhinal cortex (TEC) is a primate-specific transition zone between the entorhinal allocortex and the temporal isocortex. Neurons in the lamina pre-alpha of TEC are known to be the first to develop intraneuronal changes in the course of Alzheimer's disease. In order to shed light on this important feature, we studied as yet unknown morphological and neurochemical characteristics of the TEC of the African green monkey (Cercopithecus aethiops sabaeus). Using light- and electron-microscopic immunocytochemistry, the distribution and morphology of neurons containing calcium-binding proteins were described and compared with those in the adjacent cortices. Light-microscopic analysis revealed that parvalbumin-containing neurons were distributed in all cortical layers. Calbindin-containing cells were fewer but also present in each layer. Calretinin-containing neurons were largely confined to the upper layers of the TEC. All three types of neuron showed pyramidal-like, multipolar and bipolar shapes; their dendrites were smooth or beaded. Ultrastructural studies revealed immunopositive somata with infolded nuclei and large amounts of cytoplasm. The somata were only sparsely innervated by symmetric synapses. Immunopositive dendrites were almost exclusively covered with immunonegative axon terminals establishing symmetric and asymmetric synapses. Immunopositive terminals established symmetric contacts with immunonegative dendrites and somata. Only occasionally, could synaptic contacts between immunopositive pre- and postsynaptic structures be observed. The comparison of neurons in the TEC and adjacent cortices revealed no striking differences. In summary, the morphological and neurochemical characteristics of TEC neurons as analyzed in our study do not provide an explanation for the early onset of neurodegenerative changes in the TEC.