1. Twenty-four-hour intra-arterial blood pressure recordings were made before and after chronic β-receptor blockade in 34 patients with essential hypertension.
2. Subjects were randomized in double blind fashion to either atenolol, metoprolol, pindolol or slow-release propranolol.
3. Drugs were administered in a once-daily variable dose regimen for a period of 3–8 months (mean 5.0 ± sd 1.4).
4. All four drugs reduced blood pressure significantly 24 h after the last dose but there were considerable differences in control when each intervening hour was assessed separately.
5. Not all β-receptor blockers in conventional formulations are equally effective as antihypertensive agents over 24 h when taken once daily.