Reports suggest that patients with Alzheimer's disease have deficient olfactory sensitivity and disruption of the cholinergic pathways. Elucidation of the cholinergic system in the olfactory bulb is essential to the understanding of this disease for diagnostic and therapeutic advances. Possible cholinergic projection to the olfactory bulb was studied in rats with transected medial forebrain bundles by stimulating the horizontal limb of the diagonal band, which contains cholinergic perikarya. Electrical stimulation of the horizontal limb of the diagonal band evoked negative potentials in the granule cell layer. Neurons in the granule cell layer were excited, and those in the mitral cell layer and external plexiform cell layer showed inhibition, often followed by excitation. These responses were attenuated by the administration of the cholinergic blocker, atropine. The role of cholinergic projection to olfactory bulb neurons is discussed.