The distribution of serotonin immunoreactivity in the main olfactory bulb (MOB) of the Mongolian gerbil (Meriones unguiculatus) was examined by immunohistochemistry. Seven distinct layers of the Mongolian gerbil MOB-stained with cresyl violet were identified. Serotonin-immunoreactive (IR) cell bodies were not found in the MOB. The serotonin-IR nerve fibres had a specific laminar distribution and morphology in the gerbil MOB. Serotonin-IR nerve fibres were observed in the glomerular, external plexiform and granule cell layers of the MOB. These serotonin-IR nerve fibres showed varicosities that were larger than the thickness of the axon. The highest density of serotonin-IR nerve fibres was in glomeruli of the glomerular layer. The average fibre density in the glomerular layer was more than three to four times the density in the infraglomerular layers. Glomerular serotonin-IR fibres were much more intensively stained than infraglomerular serotonin-IR fibres. This result suggests that serotonin-IR nerve fibres of Mongolian gerbil MOB are extrinsic and may act to modulate the olfactory transmission.