The study aimed to : - To determine whether there are significant differences between Fartlek training and the increase in the molecular biology (VEGF - basal fibroblast growth factor) for arena and field players in an (800m) youth event. - Determine whether there are statistically significant differences between Fartlek training and the increase in molecular biology and some physical variables for the players (800m) in the arena and field for youth. - Determine whether the Fartlek training method is the most appropriate to achieve statistically significant differences in the research variables for the players (800m) in the arena and field for youth. The researchers used the experimental method for its suitability and the nature of the research and its procedures. The community sample was deliberately determined for the youth category, and their number was (8 players) from the specialized school, among the elite players who represented their percentage (%28) from the original community for the season (2021_ 2022), which is the method that the researchers used in the one-group tests. The duration of the training program (90 days), which included the Fartlek training method The researchers reached the following conclusions:: Fartlek training had the effect of increasing the molecular biology (vascular endothelial growth factor (VEGF)-basic fibroblast growth factor (FGF)) for track and field athletes in the event (800m) for youth. - Fartlek training had a positive effect on some physical variables, especially in the post tests of field and field players in the (800m) competition for youth. The researchers recommended a set of recommendations, the most important of which are : Use a non-fertoc training method and introduce strength exercises. Use of vascular endothelial growth factor (VEGF)-basic fibroblast growth factor (FGF) as an indicator of muscle efficiency in oxygen consumption (Vo2 max). Comparison of the aerobic and anaerobic system and its effect on the increase of vascular endothelial growth factor (VEGF) - essential fibroblast growth factor (FGF) Using the same research variables on a sample of applicants in different events and comparing the results with the youth sample.