There are numerous studies showing the effects of exercise on sex hormone concentrations. However, research analyzing the fluctuation of sex hormones in response to resistance training (RT), during different phases of the menstrual cycle (MC), is not comprehensive.   PURPOSE: The purpose of this project was to analyze the response of estradiol (E2) concentrations, when incorporating an acute RT session, during four different phases of the MC. METHODS: Untrained healthy eumenorrheic women were recruited for this study. The study was divided into two phases: preparation phase and intervention phase. The preparation phase included the initial interview, MC determination, and three familiarization sessions. The intervention phase comprised of the four exercise sessions at the midpoint of each MC phase. Each session included the completion of a twenty-four-hour dietary recall, a body composition assessment, a RT circuit, and pre and post exercise blood draw. Regarding the percent change of E2, we analyzed a total of six subjects. For the pre/post comparisons, we analyzed a total of eight subjects for the late follicular phase (LFP) and seven subjects for the other three phases. RESULTS: The ANOVA with repeated measures did not show significant differences in E2 [F (3,15) = 2.55, p=0.09, ƞ2=0.338], caloric intake [F(3,18) = 51.42, p = 0.00, ƞ2 = 0.896], proteins [F(3,18) = 0.88, p = 0.47, ƞ2 = 0.128], fats [F(3,18) = 0.93, p = 0.45, ƞ2 = 0.135], and carbohydrates [F(3,18) = 2.47, p = 0.10, ƞ2 = 0.292] between the four phases. A one-tailed paired sample t-test revealed pre- to post- E2 concentrations were significantly higher at LFP [t(7)=6.117, p<0.000] and LLP [t(6)=2.624, p=0.020]. No statistically significant differences were found for EFP [t(6)=0.817, p=0.223] or ELP [t(6)=0.585, p=0.290]. Statistical power was low, and the effect size was large. CONCLUSION: No significant changes were found in pre and post percent change in RT exercise between phases. However, significant increase in E2 concentrations were found after an acute session of RT in the LFP and LLP.