AbstractThe internal differentiation of the interpeduncular nuclear complex was determined by quantitative cytoarchitectonic analysis in specimens from each of the suborders of Rodentia: rat (Rattus) of the Myomorpha; guinea pig (Cavia), chinchilla (Chinchilla), and capybara (Hadrochoerus) of the Caviamorpha; and the mountain beaver (Aplodonia) from the Sciuromorpha. In each of these rodents the constituent cells of the complex ae grouped into four spatially distinct subnuclei: pars lateralis (PL), pars medialis (PM), pars dorsalis magnocellularis (PDM), and pars dorsalis parvocellularis (PDP). There is a fifth subnucleus, the paramedian, inChinchilla. Sample distributions, and means, of cell sixes were measured in each of these subdivisions in each species; and cell densities of each subdivision were determined inRrattus. In all these animals PD had the smallest cells. PDM had the largest cells, except that inHydrochoeruscells of PL, and inCaviacells of PM, were as large as those of PDM. InRattusandAplodontiaPL and PM were not different in cell size, and inRattustheir cells were of the same density. These findings cast doubt upon criteria previously used to subdivide this nuclear complex, and provide a consistent and replicable classification of its components.