ABSTRACT: The distribution of blood leukocytes in the preovulatory follicle (PO) and in the corpus luteum (CL) of the pig was studied by light microscopy. The number of macrophages increased in the freshly luteinized follicle, decreased subsequently in the developing and mature CL, and then increased again in the regressing CL. However, the relative proportion of macrophages as a percent of total blood leukocytes present did not change throughout the cycle. The increase in the number of lymphocytes was the greatest increase after ovulation as compared to all the other blood leukocytes observed. Even though the number of lymphocytes decreased thereafter, their numbers expressed as a percent of total blood leukocyte number remained high in the developing and mature CL. Eosinophils were the most prominent blood cell type present in thecal tissue of PO and in the regressing CL. In other stages of the CL, few eosinophils were observed. Neutrophil numbers remained moderate and unchanged throughout most of the estrous cycle except in the regressing CL, where the number of neutrophils was slightly increased. Low numbers of plasma cells were observed in all structures studied and no significant changes due to stage were apparent. The distribution of lymphocytes in the corpus hemorrhagicum (CH) and eosinophils in the regressing CL was different in different regions of the ovarian structure. In summary, blood leukocytes, most notably macrophages, lymphocytes and eosinophils, differentially migrate into specific structures of the ovary at specific stages of the estrous cycle of the pig. The possible involvement of these blood leukocytes in the modulation of ovarian events is discussed.