The neurons of the medial cerebellar nucleus of the rat were studied with several variants of the Golgi technique. Using the criteria of somatic size, dendritic pattern and location, seven cell types were designated within the nucleus. Large multipolar neurons were most prominent in dorsal portions of the nucleus while small multipolar and fan-shaped neurons predominate ventrally. The rostral and caudal halves of the medial nucleus differ in their neuronal orientation. There is a caudolateral to rostromedial cellular orientation in the caudal portion of the nucleus, and a caudomedial to rostrolateral orientation in the rostro-ventral part of the medial nucleus. The large multipolar neurons of the rostro-dorsal region display no special orientation. In addition to these rostral and caudal architectural differences, there are several longitudinal zones within the medial nucleus previously reported in Nissl preparations (Beitz & Chan-Palay, 1978). The medial and lateral elliptical zones are comprised predominantly of neurons of the nuclear boundary, fan-shaped neurons and small bipolar neurons. The round cell central zone, by contrast, is comprised primarily of small and large multipolar neurons and bouquet neurons. We conclude that the medial cerebellar nucleus consists of several distinct regions each with a characteristic arrangement of its constituent cells and that these regions correspond to those previously reported as having different physiological functions.