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Behavioral, electrophysiological, and anatomical assays of non-human primates have provided substantial evidence that the hippocampus and dentate gyrus are essential for memory consolidation. However, a single anatomical and stereological investigation of these regions has been done in New World primates to complement those assays. The aim of the present study was to describe the cyto-, myelo-, and histochemical architecture of the hippocampus and dentate gyrus, and to use the optical fractionator method to estimate the number of neurons in the hippocampal pyramidal and granular neurons in the dentate gyrus of the Cebus monkey. NeuN immunolabeling, lectin histochemical staining with Wisteria floribunda agglutinin (WFA), enzyme-histochemical detection of NADPH-diaphorase activity and Gallyas silver staining were used to define the layers and limits of the hippocampal fields and dentate gyrus. A comparative analysis of capuchin (Cebus apella) and Rhesus (Macaca mulatta) monkeys revealed similar structural organization of these regions but significant differences in the regional distribution of neurons. C. apella were found to have 1.3 times fewer pyramidal and 3.5 times fewer granular neurons than M. mulatta. Taken together the architectonic and stereological data of the present study suggest that hippocampal and dentate gyrus neural networks in the C. apella and M. mulatta may contribute to hippocampal-dentate gyrus-dependent tasks in different proportions., (Copyright (c) 2010 Elsevier B.V. All rights reserved.)