The distribution of immunoreactive interleukin-1 receptor type I (IL-1R tI), IL-1 alpha, and IL-1 beta, and of macrophages, was investigated immunohistochemically in the mouse ovary during follicular growth, ovulation, and luteinization. For this purpose, an indirect immunofluorescence technique, using specific monoclonal antibodies against mouse IL-1R tI, mouse IL-1 alpha, IL-1 beta, and macrophage antigens (CD11b/CD18) was used with sections of paraffin-embedded ovaries from eCG and eCG/hCG-treated 12-wk-old B6C3F-1 female mice. During follicular development, IL-1 alpha, IL-1 beta, and IL-1R tI staining were confined to the theca-interstitial layer of growing follicles with one remarkable exception. Intense IL-1R tI still staining was present in the cytoplasm and plasma membrane of the murine oocyte. During ovulation, IL-1 alpha and IL-1 beta were still confined to the theca layer, but faint IL-1R tI staining was initiated in cumulus cells and in granulosa cells just before follicle rupture. Immediately after follicle rupture, granulosa cells stained positive for IL-1R tI, IL-1 alpha, and IL-1 beta. During luteinization, granulosa-luteal cells of the corpus luteum demonstrated strong IL-1R tI, IL-1 alpha, and IL-1 beta staining. Macrophages were detected in the theca layer and stroma, but never within the follicle before ovulation. Immediately after ovulation, there was a rapid entry of macrophages into the follicle, and macrophages were also present inside the corpus luteum. Our morphological results support a possible autocrine-paracrine role of the mouse ovarian IL-1 system in ovulation and luteinization.