1. In 27 severe primary hypertensive patients nifedipine (10 mg), administered orally, induced prompt (–21% of control at 30 min) and persistent (–17% at 120 min) fall of mean arterial pressure mediated through reduction of peripheral vascular resistance with rise of cardiac output. 2. The sublingual route (nine cases) showed more rapid onset of action and equal antihypertensive effectiveness. 3. In five patients with hypertensive crisis and acute left ventricular failure, the drug strikingly reduced systemic and pulmonary arterial pressures and relieved pulmonary oedema. 4. Prompt efficacy, ease of administration, absence of important side effects indicate that nifedipine may be a useful therapeutic agent in severe hypertension and in critical conditions that require rapid lowering of blood pressure.