Abstract: Aim\ To study the relationship between plasma nitric oxide and the pathogenesis of essential hypertension .\ Methods\ Plasma nitric oxide were measured in 50 patients with essential hypertension before and after treatment and in 36 control subjects.\ Results\ （1）The plasma nitric oxide level was reduced markedly in hypertensive patients than in control subjects（P＜001）.（2）The patients of stage Ⅰ had highest plasma nitric oxide level, while the stage Ⅱ had moderate,and the stage Ⅲ the lowest（F=67.58,P＜0.01）.（3）The level of nitric oxide in severe hypertensive patients was lower than thosse of mild and moderate hypertensive patients（P＜001）.（4）The leve; of nitric oxide was lower in hypertensive patients with congestive heart failure than in those without congestive heart failure（P＜001）.\ Conclusion \ There were significant relationship between plasma nitric oxide and the severity of hypertension. It might served as an index to assess the severity of disease in patients of essential hypertension.