Luteal regression is initiated by prostaglandin F_2α (PGF_2α). In domestic species and primates, demise of the corpus luteum (CL) enables development of a new preovulatory follicle. However, during early stages of the cycle, which are characterized by massive neovascularization, the CL is refractory to PGF_2α. Our previous studies showed that endothelin-1 (ET-1), which is produced by the endothelial cells lining these blood vessels, plays a crucial role during PGF_2α-induced luteolysis. Therefore, in this study, we compared the effects of PGF_2α administered at the early and mid luteal phases on ET-1 and its type A receptors (ETA-R) along with plasma ET-1 and progesterone concentrations, and the mRNA levels of PGF_2α receptors (PGF_2α-R) and steroidogenic genes. As expected, ET-1 and ETA-R mRNA levels were markedly induced in midcycle CL exposed to luteolytic dose of PGF_2α analogue (Cloprostenol). In contrast, neither ET-1 mRNA nor its receptors were elevated when the same dose of PGF_2α analogue was administered on Day 4 of the cycle. In accordance with ET-1 expression within the CL, plasma ET-1 concentrations were significantly elevated 24 h after PGF_2α injection only on Day 10 of the cycle. The steroidogenic capacity of the CL (plasma progesterone as well as the mRNA levels of steroidogenic acute regulatory protein and cytochrome P450_scc) was only affected when PGF_2α was administered during midcycle. Nevertheless, PGF_2α elicited certain responses in the early CL: progesterone and oxytocin secretion were elevated, and PGF_2α-R was transiently affected. Such effects probably result from PGF_2α acting on luteal steroidogenic cells. These findings may suggest, however, that the cell type mediating the luteolytic actions of PGF_2α, possibly the endothelium, could yet be nonresponsive during the early luteal phase.