The distribution and characteristics of serotonin-immunoreactivity in the olfactory bulb of the laboratory shrew (Suncus murinus, insectivore) was studied immunohistochemically. Serotonergic neurons were found only in the subependymal layer of the main olfactory bulb. These neurons were 8-12 microm in size and bipolar in shape. These serotonergic neurons had smooth nerve fibers which innervate blood vessels located mainly in the subependymal layer of the main olfactory bulb. On the other hand, other serotonergic nerve fibers with varicosities, which must be extrinsic, were detected in most olfactory layers except the olfactory nerve layer. This result suggests that intrinsic serotonergic neurons may control blood vessels and varicose serotonergic nerve fibers may act to modulate the olfactory transmission.