Nuclear morphology is known to distinguish two classes of medium-sized neurons in the neostriatum, spiny projection neurons which have an unindented nucleus and aspiny interneurons which exhibit enfoldings of their nuclear envelope. The frequency and distribution of medium-sized neostriatal neurons with indented nuclei were examined in the light microscope in semi-thick sections. Results showed that the proportion of neostriatal neurons with nuclear indentations was greater in the monkey (23%) than in the mouse and rat (4-5%). Also, nuclear enfoldings were found in more neurons of the monkey caudate than putamen, whereas in the rodent regional differences were not observed. Findings suggest that aspiny interneurons play a greater role in the integrative and functional organization of the neostriatum in the primate than in the rodent brain.