An original pathway of centrifugal acetylcholinesterase-positive fibres is described in the olfactory bulb of the rat. A dense network of positive fibres spreads out superficially at the boundaries of the lateral olfactory tract and the glomerular layer. These labelled fibres converge towards atypical glomerular structures lying close to the classical olfactory glomeruli. The atypical glomeruli are located dorsally at the medial border of the accessory olfactory bulb, in the area previously described as the "modified glomerular complex", and in the ventrolateral bulbar area. They structurally differ from typical glomeruli, as suggested by observations on semithin sections. The ultrastructural distribution of acetylcholinesterases into axonal and dendritic profiles, around and inside atypical glomeruli, is consistent with the hypothesis of centrifugal modulatory influences at this level. This study illustrates several new aspects of morphofunctional heterogeneity in the olfactory system. The glomerular layer of the main olfactory bulb can no longer be considered as morphologically and functionally uniform. Atypical glomeruli located in the mediodorsal and the ventrolateral boundaries of the glomerular layer are characterized by both structural features and an uncommonly high convergence of acetylcholinesterase-positive centrifugal fibres. Such areas might be involved in the processing of specific olfactory signals as demonstrated elsewhere for the "modified glomerular complex".