The objective of this experiment was to assess the concentrations of sulfadiazine and trimethoprim in the blood and endometrium of nonpregnant mares after oral treatment. We hypothesized that the potentiated sulfonamide would reach tissue concentrations greater than the minimum inhibitory concentration (MIC) reported for common pathogens. Over two breeding seasons, mares in estrus were treated with sulfadiazine-trimethoprim (Equisul-SDT Aurora Pharmaceutical, LLC, Northfield, MN), 333 mg/67 mg combination per mL, at a dosage of 24 mg/kg, orally, every 12 hours for five treatments. Blood was obtained at 0, 12, 36, and 60 hours. An endometrial biopsy was also performed at 60 hours. In year 1, the mean concentrations of sulfadiazine and trimethoprim at 60 hours were 12.14 μg/mL and 0.25 μg/mL in the blood and 3.19 μg/g and 0.69 μg/g in the endometrium, respectively. In year 2, the mean concentrations of sulfadiazine in the blood were 5.17, 10.22, and 13.39 μg/mL and 0.04, 0.15, and 0.27 μg/mL for trimethoprim at 12, 36, and 60 hours, respectively. Mean concentrations of sulfadiazine and trimethoprim in the endometrium at 60 hours were 7.96 μg/g and 0.23 μg/g, respectively. Concentrations of sulfadiazine and trimethoprim in the endometrium after five consecutive treatments with the oral suspension were above the in vitro MIC reported for common pathogens known to cause bacterial endometritis, for example, Streptococcus equi subsp. zooepidemicus (MIC = 0.25–4 μg/mL) and Escherichia coli (>0.25–4 μg/mL). The oral suspension of sulfadiazine-trimethoprim should be an efficacious and viable treatment for bacterial endometritis.