Recently, we showed that leukotrienes (LTs) regulate ovarian cell functionin vitro. The aim of this study was to examine the role of LTs in corpus luteum (CL) function during both the estrous cycle and early pregnancyin vivo. mRNA expression of LT receptors (BLTfor LTB₄andCYSLTfor LTC₄), and 5-lipoxygenase (5-LO) in CL tissue and their localization in the ovary were studied during the estrous cycle and early pregnancy. Moreover, concentrations of LTs (LTB₄and C₄) in the CL tissue and blood were measured.5-LOandBLTmRNA expression increased on days 16–18 of the cycle, whereasCYSLTmRNA expression increased on days 16–18 of the pregnancy. The level of LTB₄was evaluated during pregnancy compared with the level of LTC₄, which increased during CL regression. LT antagonists influenced the duration of the estrous cycle: the LTC₄antagonist (azelastine) prolonged the luteal phase, whereas the LTB₄antagonist (dapsone) caused earlier luteolysisin vivo. Dapsone decreased progesterone (P₄) secretion and azelastine increased P₄secretion during the estrous cycle. In summary, LT action in the bovine reproductive tract is dependent on LT type: LTB₄is luteotropic during the estrous cycle and supports early pregnancy, whereas LTC₄is luteolytic, regarded as undesirable in early pregnancy. LTs are produced/secreted in the CL tissue, influence prostaglandin function, and serve as important factors during the estrous cycle and early pregnancy in cattle.