The collagen-provocated platelet nitric oxide synthase (NOS) activity by the method of 3H-labelled L-arginine was compared between 19 essential hypertensives (EH) and 21 controls, and between 13 adolescents with their both parent hypertensives (FH+) and 12 adolescents without genetic hypertensive predisposition (FH-) as well. Results showed that the platelet NOS activity was lower significantly in EH group (4.76 +/- 2.01 vs 8.09 +/- 2.36 pmol.g-1.min-1, P < 0.001) and in FH+ group (3.64 +/- 2.07 vs 5.51 +/- 2.13 pmol.g-1.min-1, P < 0.05) comparing with their control groups respectively. It suggests that the inherited defect of some anti-hypertensive mechanisms like NO/NOS system may be implicated in the development of EH, which can be taken as a new "genetic marker" for detecting of specific clinic subtype. An useful data was thus presented of value in screening referred genes, for early prevention and rational remedy in essential hypertension.