- The non-stimulant cardioselective beta adrenocepter antagonist atenolol has been studied in volunteers in order to define its pharmacokinetic characteristics. Atenolol (100 and 200 mg orally) is rapidly absorbed, reductions in heart rate and systolic pressure being observed in 30 min. The effect persists for up to 8 h. Over 85% of an intravenous dose is excreted in urine within 24 h but only 50% of an oral dose. The bioavailability of approximately 50% is due to reduced absorption. Peak blood levels are observed at 2-4 h and the half life of atenolol given orally is 5-6 h. Atenolol reduces the cardiac response to standing and head-up tilt. It does not reduce circulating levels of renin but slightly impairs the renin response to tilt. Atenolol both orally and intravenously reduces supine diastolic pressure about four hours after administration, the effect persisting for up to 24 h.