Lysosomal activity in follicles of ovaries of untreated and gonadotropic-treated immature rats was investigated using cytochemical techniques for acid phosphatase localization. The major site of acid phosphatase reaction product in ovaries of untreated immature rats was in autophagic vacuoles associated with numerous atretic follicles. Little reaction product was found in cells of intact preantral follicles. In contrast, antral follicles of rats treated with PMSG (20 IU of pregnant mare serum gonadotropin 54 h before killing) displayed more acid phosphatase reaction product than did preantral follicles, especially within their thecal steroidogenic cells. Graafian follicles of rats treated with PMSG-hCG (20 IU of PMSG plus 40 IU of human chorionic gonadotropin 54 h later and killed 16 h later) were characterized by large quantities of reaction product within both granulosa and thecal cells. However, follicular cells undergoing luteinization had more acid phosphatase reaction product than did any other cell type. The reaction product in all ovarian steroidogenic cells was associated predominantly with the active Golgi complex and the extensive agranular endoplasmic reticulum, with some reaction product within vesicles (<0.15 µm). It is concluded that gonadotropic treatment stimulates the development of an extensive lysosomal system within steroidogenic cells of ovarian follicles. Since there were no morphologic indications of degeneration within these cells, these observations suggest that the lysosomal system may play a role in ovarian steroidogenesis.