The ultrastructural organization of cholinergic afferents to the rat olfactory bulb (OB) was studied with the aid of choline acetyltransferase (ChAT) immunocytochemistry in electron microscopy. Particular attention has been paid to a subset of glomeruli characterized by a remarkably high density of cholinergic afferents. Numerous cholinergic terminals making symmetric or asymmetric synaptic contacts were observed in the periglomerular area. ChAT-labelled terminals have a diameter ranging from 0.3 to 1.5 micron and contain numerous clear agranular vesicles. Axo-somatic and axo-dendritic contacts were both observed in contact with several types of target neurons. Three types of cholinoceptive, noncholinergic neurons could be identified: periglomerular cells, superficial short-axon cells, and external tufted cells. Our results provide an anatomical substrate for the hypotheses concerning the complex effects of acetylcholine in the processing of sensory information in the olfactory bulb.