1. In eighteen subjects with mild to moderate hypertension the effectiveness of the new vasodilator drug L6150 (Lepetit) was studied. Its mechanism of action was identified by systemic and renal blood flow measurements and by estimating changes in plasma renin and aldosterone concentration and plasma volume. The reflexly induced hyperkinetic circulatory state could be completely corrected by adding propranolol or prevented by pretreatment with the latter drug.
2. Sodium retention was an invariable phenomenon. It was not due to a stimulation of the renin—angiotensin-aldosterone axis since plasma aldosterone was significantly decreased, presumably as a consequence of plasma volume expansion.