From the literature we know that Bioelectrical Impedance Analyses (BIA) has emerged as one of the most popular methods for estimating relative body fat. BIA, is relatively simple, quick, portable and noninvasive method, and can be used in diverse settings, including private clinicians' office, wellness centers, and hospitals. The Tanita body-fat analyzer is a novel device to estimate body fat, based on the principles of bioelectrical impedance. It differs from other impedance systems which use surface electrodes in that the subjects stand bare-foot on a metal sole-plate which incorporates the electrodes; hence impedance is measured through the legs and lower trunk. It is evident that BIA measurement affected from variance of body fluid, and showed difference in diurnal measurements in many studies. Purpose: The purpose of this study was to examine the effect of a medium level intensity exercise (70-75%) conducted on different times of a day on bioelectrical impedance measurements. Methods: Thirty female collage students (21.95±1.80 yr,) were participated in the study. The leg-to-leg impedance apparatus (Tanita Inc, Tokyo, Japan, and Model 300) was used for the assessment of body %fat. The participants were visited two times the laboratory in the morning and afternoon. In each visit, BIA measurement was obtained before and after 70-75% intensity exercise (20 min). Each athlete was given a set of written guidelines to adhere to before her designated testing date. The guidelines included the following: 1) no large meals 4 hr before the test; 2) no vigorous exercise 12 hr before the test; 3) empty bladder 30 minutes before the test; 4) no alcohol consumption 48 hr before the test; 5) no diuretic medications two days before the test: and 6) consumption of liquids limited to 1% of body weight 2 hr before the test. Measurement was performed in bare feet and 3 hrs after waking up. Results: The differences between before and after exercise in the morning were found %0.38 for weight, and 5,36 % for fat (p<0.001). There were also significant differences between before and after exercise 0.45% for weight, and 6.36% for fat in the afternoon. Conclusion: It was found that medium intensity exercise can be effect the BIA measurement. The exercise leads to an additive decrease in bioelectrical impedance and thus to a decrease in the calculated percentage of body fat. A BIA measurement difference in afternoon before and after exercise was found higher than morning measurements. Keyword(s): exercise/fitness, measurement/evaluation, research