Ultrastructural observations on stigmas in rat ovarian follicles have been performed by scanning electron microscopy. Stigmas were classified into two types under a dissecting microscope; extensively bulging vesicles (bleb-type stigmas) and small, flat avascular areas (flat-type stigmas). The bleb-type stigma had lost its surface epithelial cells extensively, and the flattened and densely arranged fibroblasts without fibrous structures were exposed. These fibroblasts had short, serrate, cytoplasmic projections where multivesicular structure-like granules were seen. By contrast, on the flat-type stigma a small region of exposed stroma was covered with fibrous structures. Removal of the fibrous structures by the HCl-collagenase method revealed that the exposed stroma consisted of stellate fibroblasts surrounding a small opening through which a few granulosa cells were about to discharge. When the cumulus mass was protruding, it was surrounded by the fibrous structures. These findings indicate that both the stellate fibroblast formation and the presence of fibrous structures are needed for the release of ova through the stigma.