Thesis (M.S., Animal and Veterinary Science) -- University of Idaho, December 2014 | In cattle, early embryonic loss may occur due to the premature secretion of prostaglandin F2α (PGF2α) during days 14-16 after fertilization. The objective of the first experiment in this study was to determine aspirin's effects on PGF2α during the luteal phase of the estrous cycle in lactating dairy cows by characterizing blood plasma prostaglandin metabolites (PGFM) and progesterone (P4) concentrations. Aspirin decreased PGFM for 30 hours after last bolus administration and increased the day to luteolysis. The objective of the second experiment was to determine the effectiveness of the initial gonadotropin-releasing hormone (GnRH) in a 5-day CIDR (controlled internal drug releasing inserts)-Cosynch resynchronization protocol on fertility to second insemination in lactating dairy cows. No differences in pregnancy rate per artificial insemination (PR/AI) were observed between no GnRH (treatment) and GnRH (control) for the second insemination. Indicating that GnRH may not be necessary at the initiation of a CIDR-Cosynch resynchronization protocol.