Projections from the medial accessory olive (MAO) to lobules VI, VII and VIII of the cerebellar vermis were investigated in the rat using the retrograde transport of wheatgerm agglutinin-conjugated horseradish peroxidase (WGA/HRP). When WGA/HRP was injected into the anterior half of lobule VI (VIa), retrogradely labeled neurons were found in a relatively narrow zone which traversed approximately the middle of the MAO and a portion of the nucleus beta. In contrast, injection into the posterior half of lobule VI (VIb, c) resulted in labeling of neurons in the medial and lateral portions of the MAO and in the nucleus beta. When HRP was confined to lobule VII, labeled neurons were found in the medial portion of the caudal MAO. No labeled neurons were found in the nucleus beta. When the injection was restricted to lobule VIII, labeled neurons appeared along the caudal end of the MAO. There were many labeled neurons in the nucleus beta. The central portion of the caudal MAO was free of labeled neurons unless the enzyme encroached upon a part of the anterior lobe. Thus, the projection areas to lobules VI-VIII formed a circle surrounding the free area and each of the lobules received fibers from a well-circumscribed portion: lobules VI-VIII received projections from the rostral, medial and caudal portion of the imaginary circle, respectively. The lateral portion was shared by the areas projecting to lobules VI and VIII. In addition, these lobules received projections also from an overlapping area in the nucleus beta.