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The intrauterine milieu is a complex mixture of substances originating from serum and endometrium that support blastocyst growth and development. The present study identified alterations in glucose and amino acids in response to an early rise in progesterone (P4), which accelerates blastocyst growth and development. Bred ewes received daily injections of either corn oil (CO) vehicle or P4 from 36 h postmating (Day 0) to either Day 9 or Day 12. Another group of ewes received P4 to Day 8 and the antiprogestin mifepristone (RU486) from Day 8 to Day 12. The total amount of glucose, aspartate (acidic amino acid), arginine and lysine (basic amino acids), and citrulline, asparagine, serine, glutamine, beta-alanine, and alanine (neutral amino acids) was greater in uterine flushings from early P4- than CO-treated ewes on Day 9. On Day 12, only arginine and lysine were higher in uterine flushings from P4-treated …