Based upon observations in 5 surgically proved cases of primary aldosteronism, laboratory tests were sought that might prove useful in diagnosis. The data revealed that plasma volumes were well above the normal range, in contrast to the levels seen in other forms of hypertension. In each case the low serum potassium level rose toward normal after oral administration of spironolactone, 1 gm. daily for 3 days, in contrast to the near absence of a response in patients with essential hypertension. Rigorous sodium restriction led to potassium retention. Such observations should furnish strong presumptive evidence for a diagnosis of primary aldosteronism.