Integrin α:β heterodimers mediate cell contacts to the extracellular matrix and initiate intracellular signaling cascades in response to a variety of factors. Integrins interact with many determinants of cellular phenotypes and play roles in controlling the development, structural integrity, and function of every type of tissue. Despite their importance, little is known about the regulation of integrin subunits in the mammalian ovary and how they function in folliculogenesis. To determine their relevance to ovarian physiology, we have studied the expression of integrin subunit mRNAs by Northern blot analysis and in situ hybridization in ovaries of wild-type, growth differentiation factor 9 (Gdf 9) knockout, FSHβ (Fshb) knockout, and inhibin α (Inha) knockout mice. Integrin α₆ mRNA is expressed in oocytes and granulosa cells of single-layer follicles and in oocytes and theca cells of multilayer follicles. Integrin α₆ is highly expressed in Gdf 9 knockout ovaries, which are enriched in oocytes and primary (single layer) follicles because of a block at this stage of follicular development. Integrin α_v mRNA is most highly expressed in the granulosa cells of multilayer growing follicles, and therefore only low levels of expression are detectable in the Gdf 9 knockout ovaries. Integrin β₁ mRNA exhibits a broad expression pattern in ovaries, including oocytes, granulosa cells, theca cells, and corpora lutea. Integrin β₃ mRNA is expressed in theca and interstitial cells and is upregulated in corpora lutea. It is nearly undetectable in ovaries of Fshb knockout mice, which develop preantral follicles but have no luteal cells. Integrin β₅ mRNA is predominantly expressed in granulosa cells of multilayer follicles. It is expressed at high levels in the Fshb knockout mice and in a compartmentalized manner in the granulosa cell/Sertoli cell tumors that develop in the Inha knockout mice. Specific integrins are associated with ovarian cellular phenotypes in mice, which raises intriguing possibilities as to integrin functions in oocyte competence, follicular development, luteinization, and granulosa cell proliferation.