The aim of this study was to investigate possible cardiovascular changes to chosen biochemical indices regarding specific forms of exercise (soccer, swimming etc) in boys 9 - 14 years old. The concentration levels of the following biochemical parameters were measured: reactionary protein (CRP), total antioxidant capacity (T.A.C), and Fibrinogen. Moreover, body fat percentage and body mass index were measured and maximum oxygen intake (VO2 max) was estimated. The sample comprised 168 boys, aged 10.33+0.88, 10.62+0.21, 11.68+0.11 and 10.71+0.24 years old respectively, who were classified into four Groups (1, 2, 3, 4) according to their level of physical activity. Blood samples were taken from all four Groups in the morning of the same day after a 12 - hour, all - night fasting. The statistical analysis of the results (variance analysis one way anova, post hoc - Bonferroni) showed statistically significant differences (a = 0.1) in the levels of reactionary protein (CRP), fibrinogen (fib) and total antioxidant capacity (TAC). More specifically there were statistically significant differences a) regarding CRP, between Group 4 and all other Groups, b) regarding fibrinogen (fib), between Group 4 and Groups 1 and 2, and c) regarding total antioxidant capacity, between Group 4 and Group 2 as well as between Group 3 and Group 2. Additionally, statistical analysis (Pearson correlation) showed statistically significant correlations (a = 0.05) a) between body fat percentage and fibrinogen levels (positive correlation), b) between maximum oxygen intake (VO2 max) and fibrinogen levels (negative correlation) and c) between body mass index and fibrinogen levels (positive correlation). From these results, it is obvious that exercise benefits children, probably protecting their organisms against cardiovascular diseases.