Etiology:  Interstitial cell tumors are of Leydig cell origin.

Incidence:  Common in aged F344 rats (up to 80%).

Clinical Signs:  There are often no clinical signs.

Pathology:   Grossly, tumors are circumscribed, soft, lobulated, light yellow to hemorrhagic, single or multiple masses involving one or both testes.  Histologically, tumors consist of 2 types of cells arranged in sheets.  Cells are either polyhedral to elongated with granular to vacuolated cytoplasm or smaller with hyperchromatic nuclei and scanty cytoplasm.

Diagnosis:  Diagnosis can be made upon necropsy and histopathologic examination of tissue.