Leukocytes, especially neutrophils and macrophages, traditional cellular regulators of the immune system, reside within the tissue architecture of the rodent and human ovary and dramatically increase in number, in response to gonadotropin, in the theca of preovulatory follicles. Evidence strongly suggests a modulatory role for leukocytes in ovarian tissue remodeling events, such as ovulation, luteinization, and luteolysis. The present study investigates the ovarian localization and potential gonadotropin regulation of intercellular adhesion molecule-1 (ICAM-1), an important factor in neutrophil and monocyte attachment to endothelium. Reverse transcription-polymerase chain reaction and immunohistochemical detection and quantification of ICAM-1 mRNA and protein were carried out in ovaries of immature and eCG/hCG-primed rats during the periovulatory period (0, 6, 12, and 24 h post-hCG). While whole ovarian ICAM-1 mRNA levels did not vary significantly during the preovulatory period, ovarian follicles exhibited ICAM-1 mRNA and protein specifically within the thecal region, where mRNA expression increased 5-fold and protein expression increased 6-fold when comparing pre-hCG levels with those at the estimated time of ovulation (12 h post-hCG). Thecal ICAM-1 was most prevalent in highly vascularized regions as evidenced by serial staining with an endothelium-specific antibody. Granulosa layer ICAM-1 immunoactivity was acquired only during/after follicle rupture. These results show ICAM-1 is localized within the ovarian theca and its expression is associated with follicular development in periovulatory follicles, peaking in expression at the time of rupture. Additionally, ICAM-1 is expressed among granulosa-lutein cells of the ovulating follicle and developing corpus luteum. Taken together, these findings suggest rat ovarian ICAM-1 may be instrumental in the active recruitment of leukocytes into the preovulatory ovary and may have a role in corpus luteum formation.