Nuclear volume, neuronal and glial numbers, neuronal and glial density, neuronal shape and neuronal orientation were studied in the rat medial cerebellar nucleus. A statistical analysis of the data obtained from measurements of neuronal lengths, widths and areas indicated that two classes of neurons are present in the nucleus with a separation into a set of small cells and a set of larger cells occurring at the 176-199 μm2 interval. The large neurons range in width from 10 to 22 μm and in length from 18 to 42 μm, while the small cells range from 4 to 16 μm in width and from 7 to 24 μm in length. On the basis of quantitative cytoarchitectonic studies of the neurons present, four longitudinal rostro-caudal zones were defined within the medial nucleus: two zones containing elliptical cells, located on the medial and lateral aspect of the nucleus; an intervening zone containing round cells and a caudomedial zone containing a mixture of round and elliptical neurons. The significance of these sagittal zones with regard to the scheme of longitudinal zonation within the cerebellum is discussed. A comparison of the arrangement of neurons in the caudal and rostra] halves of the nucleus revealed that the caudal neuronal population displays a predominant diagonal arrangement with their perikarya oriented from caudolateral to rostromedial. The majority of cells in the rostral part of the medial nucleus, however, show either a diagonal orientation preference from caudomedial to rostrolateral or a longitudinal rostral to caudal arrangement. The results provide quantitative data concerning nuclear volume, cell density and cell shape in the medial cerebellar nucleus of the rat, and in addition, demonstrate a difference in neuronal arrangement between anterior and posterior portions of the nucleus. These two morphologically different portions of the medial cerebellar nucleus are comparable to the areas of the nucleus that have been previously demonstrated in the literature to be functionally separable.