Introduction: There is a little information about the effect of rest period length on hormonal adaptation. The purpose of this study was to investigate the effect of short vs. long rest period between the sets in resistance training on testosterone to cortisol ratio (Ts /‪ Co ratio). Material & Methods: Sixteen experienced, weight trained men volunteered to participate in the study. Subjects were randomly assigned to either a short (30 second; P30) or long (120 second; P120) rest period group. During the first 2 weeks of training, 3 sets of 10-12 repetition maximum (RM) with 120 second rest intervals between sets and exercises (hamstring with machine, squat, dead lift and leg press) were performed by both groups. During the next 6 weeks of training, the P120 group trained using 120 second between sets and exercises (4 sets of 8-10RM), and the P30 group trained using 30 second between sets and exercises as the 6 weeks of training progressed (4 sets of 8-10RM). Blood samples were taken at baseline, immediately and 30 min after the last session of training for measurement serum Ts and Co concentrations. Results: The results indicated that Ts /‪ Co ratio had tendency to increase immediately after last session of resistance training in P30 and P120 group however no significant differences were observed between these groups. The Ts /‪ Co ratio at post exercise was higher but not significance in the P120 in compare to the P30. Conclusion: Our results suggested that there is no significant difference in Ts /‪ Co ratio after short or long rest period between the sets in resistance training. تفاصيل المقالة