The postnatal development of the medial accessory olive (MAO) was studied in the rat from birth to adulthood. In newborn rats, the inferior olivary complex exhibited an adult cytoarchitectonic pattern, facilitating the precise delimitation of the MAO. Computer-assisted measurements of neuronal perikarya in 1 micron thick plastic sections revealed a 40% increase in perikaryal diameters from day of birth (PO) to the twenty-first postnatal day (P21). This growth takes place mainly during the first postnatal week, the phase of perikaryal maturation, whereas it is almost non-existent during the second week, the phase of sudden neuropil expansion. The ultrastructural study gave the following results: at P1-P5, only the neuronal perikarya have attained a certain degree of maturity. The neuropil is composed of profiles of unknown origin, among which growing dendrites are numerous, but mature synapses are scarce. By P7-P10, the cytological characteristics of the perikarya reached an adult stage. The dendrites begin to acquire their adult features by their emission of racemose protrusions and by their organization into protoglomerular formations. The most important step in the structural differentiation of the MAO was found to occur between P10 and P15. It is at this later age that the neuropil exhibits a complex neuronal organization similar to the adult, characterized by the presence of olivary glomeruli and of neuro-neuronal gap junctions. The fact that these electrotonic junctions appear a long time after the appearance of chemical synapses, indicates that the ontogeny of the MAO chemical transmission precedes electrical transmission. On P15 and thereafter, the maturation of the MAO proceeds mainly by increasing the number of synaptic connections and by glial differentiation. These structural developmental stages of the MAO were related to the different steps of functional development of the olivocerebellar system.