1. Atenolol (ICI 66.082, Tenormin) is a new beta-adrenoreceptor-blocking agent, devoid of intrinsic sympathomimetic and membrane-stabilizing properties. It does not cross the blood-brain barrier. 2. Atenolol given to hypertensive patients in initial open trials reduced arterial blood pressure significantly. 3. A double-blind comparison between atenolol and placebo in forty-five patients with essential hypertension demonstrated that atenolol gave a statistically significant reduction of blood pressure (delta 28/15 mmHg, P less than 0-005). 4. The optimum anti-hypertensive dose of atenolol in patients with mild to moderately severe essential hypertension was 200 mg daily. 5. Atenolol was compared with propranolol in thirty patients with essential hypertension. No statistically significant differences of anti-hypertensive effect were observed between the two drugs. 6. Long-term results (up to 2 years) in 117 hypertensive patients indicate that drug tolerance is good. No serious toxic effects were observed. 7. In four of twelve hypertensive patients with obstructive airways disease atenolol had to be withdrawn owing to deterioration of ventilatory function.