Insight is rapidly developing into the physiological production and function of nitric oxide. This small molecule has several physiological actions that oppose the pathogenesis of hypertension. Originating in the endothelium, it causes vascular smooth muscle relaxation, lowering blood pressure. In the central nervous system nitric oxide tonically depresses activity of the sympathetic nervous system, and in the kidney it regulates renin release and causes natriuresis. Nitric oxide may also contribute to the protective action of oestrogen.