In this study, the effects of external sodium bicarbonate (NaHCO3) intake on performance in middle-distance running with additional weight were investigated. Male endurance athletes (n=13) with an average age of 26.38 ± 2.81 (years) and 5.92 ± 1.84 (years) of sport experience, and with a maximum bodyweight of 100 kilograms are involved voluntarily. Performance measurements were made by Cooper run test, while the athletes carried a load with the additional weight of 20% of their body weight. One group of athletes were given 300 ml fruit juice mixed with 20 gr. starch and the other group was given sodium bicarbonate (NaHCO3) at a dosage of 0.3 g / kg mixed with 300 ml fruit juice given as single-blind. Anthropometric measurements, blood lactate, running distance and heart rate were measured on the test days. The analysis was examined by repetitive measures ANOVA at p<0.05 significance level via SPSS 14 program. The results revealed that running distances with the intake of sodium bicarbonate (NaHCO3) increased and there was a significant difference in blood lactate levels (p <0.05). In Cooper running test, the performance of athletes using sodium bicarbonate (NaHCO3) with the additional weight of 20% of their body weight increases.