Background and Objectives: Apolipoprotein O (apoO) is a 198 amino acids protein that exists predominantly in high-density lipoprotein (HDL). It may exert cardioprotective effects via decreasing fat accumulation and increasing removal of cholesterol from macrophages. Although the health benefits of exercise are well documented, no study has yet investigated the effects of various types of training, including resistance training on apoO level. Therefore, we aimed to determine effects of five weeks of circuit resistance training with different intensities on plasma levels of apoO, HDL, total cholesterol (TC) and triglyceride (TG) in young untrained men. Methods: Forty-five age- and weight-matched healthy untrained men were randomly assigned to a control group (n=10) and four training groups: training at 20% intensity (n=9), training at 40% intensity (n=8), training at 60% intensity (n=7) and training at 80% intensity (n=8). The subjects performed circuit resistance training at 10 stations (30 seconds at each station) with three repetitions, without rest between stations and with 3-minute active rest between the repetitions. The training was carried out for 45 minutes per session, three sessions a week, for five weeks. Venous blood samples were taken 48 hours before the first exercise session and 48 hours after the last training session. Plasma levels of apoO, HDL, TC and TG were measured using commercial kits. Data were analyzed using repeated measures ANOVA and Tukey's post hoc test at significance level of 0.05. Results: After the training intervention, mean plasma level of TC and TG did not differ significantly between the study groups (P>0.05). Training at 80% of one-repetition maximum (1RM) caused a slight decrease in the apoO concentrations. Moreover, apoO concentration was significantly higher in the 20% 1RM training group compared to other study groups (F=11.599, P<0.002). Conclusion: Our results indicate that circuit resistance training at 80% of 1RM can decre
