1. Two thousand, three hundred and twenty-eight men and 1496 women between the ages of 35 and 64 years were screened for hypertension and their plasma sodium and potassium concentrations measured. Those on antihypertensive or diuretic treatment were excluded from further analysis. 2. After adjusting for age, body mass index and other variables, plasma potassium was negatively associated with both systolic and diastolic pressure in men and women. A decrease in plasma potassium of 1 mmol/l was associated with an increase in systolic pressure in women of 7 mmHg (P less than 0.001) and diastolic pressure of 4 mmHg (P less than 0.001). In men the corresponding increases were 4 mmHg (P less than 0.01) and 2 mmHg (P less than 0.05). 3. After adjusting for the other variables as above, plasma sodium was positively related to systolic blood pressure but not to diastolic pressure. An increase in plasma sodium of 1 mmol/l was associated with an increase in systolic pressure of 1 mmHg in both men (P less than 0.01) and women (P less than 0.05).