The relationships between dosage, plasma level, and degree of β-adrenergic blockade were determined in 28 patients chronically treated with oral propranolol. The intensity of β-blockade correlated poorly with drug dosage but was linearly related to the logarithm of the plasma concentration of propranolol. Eight patients formed a distinct group whose ratio of β-blockade to plasma concentration was 2.6 times lower and whose ratio of plasma level to dosage was 2.5 times higher than those of the remaining 20 subjects. Since patients are dichotomous in their concentration-effect ratio during oral propranolol therapy, the degree of β-blockade cannot be reliably estimated from the propranolol plasma level alone.