AbstractA radioimmunoassay has been developed for propranolol with a sensitivity of 2.37nmol l−1in unextracted plasma using a 50 μl sample. Plasma concentration measurements were made on samples from volunteers for up to 8 h after they had been given 5, 10, or 40 mg of propranolol by mouth. Analysis of the results showed that mean elimination half‐lives and total body clearances were similar following each of the doses and that the area under the curve was proportional to the dose. Steady‐state propranolol concentrations in 17 patients on regular propranolol treatment were linearly related to the dose over the range 20.640 mg d−1; the regression line extrapolated to the origin. These data indicate non‐saturable kinetics for the hepatic metabolism of propranolol within the dose ranges investigated and lead us to believe that there is no ‘oral‐threshold’ dose for propranolol. The radioimmunoassay may be useful in clinical practice for monitoring plasma propranolol concentrations and for detecting pa