Transient endometritis is a normal consequence of breeding and results from uterine contamination with both semen and bacteria. The modulation of the inflammatory response with the use of isoflupredone has been proposed as efficient for the treatment of endometritis by increasing pregnancy rates. The aim of the current study was to determine the effects of isoflupredone on nitric oxide (NO) levels in uterine samples from mares susceptible to persistent postbreeding endometritis, presenting or not the infectious process. Seven consecutive estrous cycles were induced in 11 mares, being the first one used as control (no treatment). All mares were submitted to the following four treatments: treatment 1: control, treatment 2: glucocorticoid (GC) treatment (20 mg isoflupredone acetate) every 12 h, for three consecutive days, treatment 3: infected treatment (intrauterine infusion of 1x109 CFU/ml Streptococcus equi subsp. zooepidemicus), treatment 4: combination of GC + infected treatment (infusion of bacteria 24 h after the first GC treatment). At 12 h after the end of each treatment, uterine samples were collected by flushing and NO was determined. After nitrate reduction, total nitrite was determined by spectrophotometer. No significant differences on nitric oxide concentration were verified by analysis of variance in the different experimental groups. It is concluded that the use of isoflupredone did not alter the nitric oxide concentration in uterine flushing’s from susceptible mares 12 h after treatment