Ovaries and adjacent parovarian (mesovarial) tissues of CD-1 mice of various ages were examined to characterize cystic and proliferative epithelial lesions of the ovary and parovarian tissues. Ovaries and adjacent tissues from 6 mice each at approximately 3 and 8 months of age were processed for light microscopy and step-sections (50-micron intervals) of the entire tissue were examined. Tissues were collected from 40 mice each at 16 through 24 months of age and 3 step sections per mouse were examined. Mesonephric duct remnants were found in all mice at 3 and 8 months. Ducts were usually in the mesovarial adipose tissue and near or within the ovarian hilus and were often loosely associated with smooth muscle of the mesovarial ligament. The epithelium of the ducts varied from low cuboidal to columnar, occasional individual cells were ciliated, and small papillary configurations of epithelium were occasionally present. Ducts were dilated (> 1 mm) more often in mice at 8 months compared to 3 months, and some were continuous with cystic spaces within the ovaries. As mice aged (16-24 months), dilation and cystic change in ducts and associated compression of ovarian tissue away from the ovarian hilus became common. The epithelium of dilated ducts was generally flattened, but foci of cuboidal to columnar epithelium and/or occasional ciliated cells were present. Many ducts contained foci of hyperplastic and/or hypertrophic epithelium, and papillary projections of epithelium were occasionally found. Some of the latter lesions were consistent with a diagnosis of papillary cystadenoma. Hyperplasia of associated fibromuscular stroma was limited to a few apparently extraovarian ducts. The results of this study indicate that remnant mesonephric ductular structures are common in and adjacent to the ovaries of CD-1 mice. As mice age, these ducts become dilated and, in some, the epithelium becomes hyperplastic and/or hyperptrophic. These mesonephric duct remnants appear to be a common source of ovarian and parovarian cyts cysts and epithelial neoplasms of the ovary in mice.