Fine structure of the vomeronasal organ (VNO) was examined in an insectivore, house musk shrew (Suncus murinus), from a viewpoint of comparative anatomy. The VNO was a pair of tubular structure, about 6mm in length, and situated at the base of the nasal septum. The medial wall of the lumen of the VNO was lined with the sensory epithelium (SE) consisting of sensory, supporting and basal cells, while the respiratory epithelium (RE) lining the lateral wall was pseudostratified and covered with microvilli. Jacobson's glands were distributed on the dorsolateral to ventrolateral side of the VNO and opened to the lumen in the transitional region from the RE to the SE. Their secretion was PAS-positive bu talcian blue-negative. Ultrastructurally, sensory cells of the SE were bipolar neurons and covered on their free surface with conspicuously long microvilli. The other ultrastructural features in sensory, supporting and basal cells were similar to those in previously reported species. The ultrastructural features of the RE were also similar to those in previous reports except that the free surface was covered with microvilli instead of cilia. In addition, a few kinds of migrating cells were often observed in both the SE and the RE. Acinar cells of Jacobson's glands possessed a round to elliptical nucleus and many large secretory granules, about 1,000 nm in diameter. Rough endoplasmic reticulum and Golgi apparatus were well developed in their cytoplasm.(ABSTRACT TRUNCATED AT 250 WORDS)