Prostaglandin F[subscript]2[alpha] (PGF) is the purported luteolytic substance of domestic livestock species, including the pig. Its mechanism of action, however, is not clearly understood. Leukocytes and/or cytokines may be important mediators of the luteolytic effects of PGF on pig corpora lutea (CL). The primary focus of the research presented in this dissertation was to examine the role of macrophages in the luteolytic process in pigs. The temporal association between functional luteolysis (as determined by a decrease in progesterone concentration within individual CL) and the infiltration of macrophages into CL during PGF-induced luteolysis was examined. Prostaglandin F[subscript]2[alpha]-induced luteolysis was accomplished by injecting individual, steroidogenically active CL on the pig ovary with a quick curing silastic implant containing PGF. Although macrophage numbers increased in PGF-treated CL prior to the induced decline of progesterone, the greatest number of macrophages were found in these regressing CL after a decline in progesterone concentration had occurred. Moreover, although a significant macrophage influx was observed in CL which had been injected with implant material alone, these CL failed to undergo early regression. It is unlikely, therefore, that macrophages play a role in induction of functional luteolysis of the pig CL. In contrast, the infiltration of macrophages into CL after a decrease in luteal progesterone concentration had occurred is consistent with a role for macrophages in structural regression of the pig CL;A primary cytokine of macrophages, tumor necrosis factor (TNF), was also measured in pig CL from cyclic and pregnant pigs by western blot analysis and immunocytochemistry in order to (1) verify the presence of TNF in the pig CL during different reproductive states and (2) determine which cells of the CL were associated with TNF. Tumor necrosis factor was present in all CL examined and the intensity of TNF immunostaining as detected by western blot analysis did not differ between steroidogenically active CL collected from nonpregnant and pregnant pigs. Immunocytochemical analysis confirmed TNF to be associated with endothelial cells of CL. The presence of TNF in steroidogenically active, highly vascularized CL, together with its cellular localization to endothelial cells is consistent with a role for TNF in the regulation of vascular development of the pig CL.