1 The antagonist activity of atenolol, metoprolol and propranolol in the CNS was estimated by determining the extent to which the drugs occupy animal r3l-and f32-receptors in CSF ex vivo at the time of lumbar puncture. 2 Five CSF and plasma samples were obtained 4 h after drug intake from subjects treated for hypertension with atenolol, 100 mg once daily and five from subjects treated with metoprolol, 50 mg three times daily. Twenty-four samples were obtained 1, 2, 4 or 12 h after drug intake from subjects receiving a single 40 mg dose of propranolol.3 The receptor occupancy in the samples was determined by adding ,3l-receptors of rabbit lung and P2-receptors of rat reticulocytes into the samples and labeling the receptors with a nonselective ,3-adrenoceptor antagonist, (-)-[3H]-CGP-12177. 4 Atenolol and metoprolol occupied, as expected, larger fractions of ,l-than ,82-receptors in CSF and plasma samples. The receptor fraction occupied by atenolol in CSF was significantly (P < 0.05) lower than that occupied by metoprolol. The differences in occupancy between the drugs in plasma, however, were not statistically significant.5 Propranolol occupied larger fractions of 132-than ,3l-receptors in the samples.Although propranolol concentrations in CSF were only 1/20-1/40 of those in plasma, the receptor occupancy of propranolol in CSF was similar to that in plasma. The differences in the extent of receptor occupancy at the various time points were not statistically significant (ANOVA for the differences, P > 0.05), because of the high variation in the occupancy at 1 and 12 h following the dose. 6 The 40 mg dose of propranolol occupied the largest, metoprolol therapy the intermediate and atenolol therapy the least fraction of P2-receptors in CSF at 4 h after drug intake.The order for receptor occupancy was thus identical to the reported order for the incidence of CNS side effects.7 Thus, we can quantitate accurately the 13l-and ,82-antagonist activity of atenolol, metoprolol and propranolol in CSF. The data on how these drugs occupy animal ,8-receptors in CSF ex vivo may be of value in analyzing their CNS-related effects.