Background & Aims: Brain-derived neurotrophic factor (BDNF) is one of the major causes of neurogenesis and neuronal plasticity which come from exercise. The aim of this study is to find an exercise which has the most influence in the least time. On the basis of this aim, the effect of short-term, three and five days of continuous endurance training (CET) and high intensity interval training (HIIT) on the serum BDNF levels in the rats were investigated.Materials & Methods: First, 50 two-month aged rats with the weight of 273±27 of Sprague Dawley race were selected and divided into four groups (CET 3 days training, CET 5 days training, HIIT 3 days training, HIIT 5 days training and control) randomly. Based on their groups, rats performed a 3-days or 5- days CET or HIIT trainings. At the end of the study, blood samplings were taken from subjects. Data have been analyzed by using two-way ANOVA.Results: The results showed that the interaction effect resulted from the increasing of serum BDNF in 5-days group in comparison with other groups of 3-days CET (p=0.039), 5-days HIIT (p=0.003), 3-days HIIT (p=0.001) and control group (p=0.003) was significant. However, the interaction effect between other groups showed no changes.Conclusion: According to results, it can be said that BDNF is influenced by the overall training time and by the increasing of time and days of performing trainings, BDNF increased. The minimum time to increase serum BDNF is at least 5 days of CET trainings.