Endothelial function is an early and sensitive marker of subclinical increase of BP in children and adolescents. It is associated with an imbalance of the key vasoactive factors (NO, endothelin-1, and serotonin). Immature spontaneously hypertensive rats (SHR line) are characterized by increased plasma concentrations of NO and endothelin-1 (by 14.7% and 2.9 times, respectively) and increased serotonin content in the plasma and platelets (by 2.7 and 2.3 times, respectively) in comparison with Wistar-Kyoto rats. Platelet count in the blood of SHR rats is by 50% higher than in Wistar-Kyoto rats.