This study was conducted to analyze the expression pattern of inhibitor of DNA binding proteins (Id)1 and Id2 mRNA on folliculogenesis in rat ovary. The ovaries were obtained from 27 days old Sprague-Dawley rat, fixed, dehydrated, and paraffin embedded. For in situ hybridization, anti-sense and sense Idl and Id2 cRNA probes were prepared and applied to the ovarian section. The ovarian sections were coated with NTB-2 emulsion. After that, the slides were developed and counterstained with hematoxylin and eosin staining. In oocytes, the hybridizational signals of Id1 mRNA were strong in primordial and primary follicles, however, there were no signals in that of atretic or preovulatory follicles. The Id2 mRNA signals were also strong in the oocytes of primordial, primary and secondary follicles. Interestingly, the Id2 mRNA was expressed specifically granulosa cells, but nor in oocyte or theca cells in dominant and preovulatory follicles. Based on these results, Id1 and Id2 mRNA was expressed specifically at follicle stages and follicular tissue and might be closely related with follicle development.