1. The β1-adrenoreceptor-blocking agent atenolol was studied in the treatment of twelve out-patients with essential hypertension.
2. With a mean dose of 110 mg of atenolol daily (range 75–200 mg/day) we observed a pronounced decrease in blood pressure.
3. Only minimal side effects were seen.
4. Cardiac output decreased from 4·6 to 3·4 l/min during treatment. This decrease did not correlate with the decrease in blood pressure but correlated well with the changes in calculated total peripheral resistance.