Possible adrenal and autonomie mechanisms contributing to the onset of essential hypertension were studied in 18 men selected from the upper and lower extremes of blood pressure distribution within a larger population. In eight of the nine pairs of subjects, who were matched for age and obesity, those with higher pressure had significantly higher resting levels of both free adrenaline and noradrenaline sulphate in plasma than their lower pressure counterparts. The higher pressure group showed a positive correlation between diastolic blood pressure and bothfree and total noradrenaline levels (r = 0.77, P < 0.05, and r = 0.81, P < 0.01, respectively). In those with lower pressure, systolic blood pressure correlated closely with plasma adrenaline (r = 0.92, P < 0.001). Increased adrenal medullary activity and altered autonomie tone appear to be features of the higher range of normal blood pressure and may precede the onset of essential hypertension.