1. The rate (k) of initial efflux of noradrenaline from platelets was determined in 63 individuals. A highly significant correlation was found between diastolic blood pressure and efflux rate.
2. When platelets are incubated in buffers with various Na+ concentrations in the range 110–170 mmol/l a higher Na+ concentration will give a faster efflux of noradrenaline for each concentration tested.
3. The value for k was determined in 41 normotensive first-degree relatives of hypertensive individuals and 21 persons with no family history of hypertension. Efflux rate of noradrenaline was significantly higher in the relatives and within this group was a subgroup with very high k values.