Distribution patterns of intermediate filaments, vimentin, smooth muscle actin, and desmin were studied in the ovine ovary using an immunohistochemical method. Vimentin was consistently expressed in follicular cells of primary, secondary, and antral follicles. Stromal cells of cortex and medulla and those surrounding the corpus luteum were immunostained with the anti-vimentin antibody. Endothelial cells lining blood vessels showed strong anti-vimentin positivity. Smooth muscle cells positive for smooth muscle actin were incorporated in the cortical region in the theca interna and externa and formed incomplete spheres around large antral follicles. In atretic follicles, the presence of positive smooth muscle cells inside follicular spaces were related with capillaries. Corpora lutea were surrounded by a layer of positive smooth muscle cells. In the hilus, smooth muscle actin was localized in the wall of blood vessels. In cortical regions, desmin-positive cells were randomly distributed and occasionally formed ill-defined clusters around tertiary follicles, but not around secondary follicles, whereas desmin was expressed in the medullary region in blood vessels. The distribution pattern of intermediate filaments in sheep ovary suggests a possible role of smooth muscle cells in the mechanism of ovulation.