Transient endometritis after breeding is necessary for clearance of bacteria and spermatozoa; however, in a subpopulation of mares, the inflammation fails to resolve in a timely fashion. The objective of this study was to describe the uterine inflammatory response in mares susceptible or resistant to persistent breeding-induced endometritis (PBIE) during the first 24 h after induction of uterine inflammation.Twelve mares were classified as susceptible (nZ6) or resistant (nZ6) to PBIE. Mares were inseminated over five estrous cycles and endometrial biopsies were collected at one time point per cycle before (0) and 2, 6, 12, and 24 h after insemination. qPCR analysis for IL1B, IL6, IL8, IFNG, TNF (TNFA), IL10, and IL1RN was performed, and endometrial inflammatory cells were counted for each sample. Relative quantification values reported fold changes in mRNA expression from 0 h values. A general pattern of expression post insemination was observed in both groups of mares. Cytokine mRNA increased at 2 h, peaked between 2 and 12 h, and then decreased.Differences were detected between groups of mares 6 h after challenge; resistant mares had higher mRNA expression of IL6, IL1RN,and IL10 than susceptible mares. Susceptible mares had an increased number of polymorphonuclear neutrophils in the endometrium 2 and 12 h after breeding when compared with resistant mares. These findings describe an inherent difference in the initial immune response to insemination and may help explain the transient nature of inflammation in resistant mares, whereas susceptible mares develop a persistent inflammation.