Propafenone, (S)- is an enantiomer Propafenone with anti-arrhythmic and beta-adrenoceptor antagonist activities. In humans (S)-Propafenone administration resulted in a significant reduction of systolic blood pressure and the rate pressure product without any significant effects on heart rate, diastolic blood pressure, PR interval, QRS duration, and QT and QTc values. The difference between the effects of (R)- and (S)-propafenone on systolic blood pressure and rate pressure product was significant. The results from the radioligand binding studies indicate that (S)-propafenone exerts beta-blocking activity in the administered dose. This is further supported by a biochemical assay showing (S)-propafenone to be 54 times more potent than the (R)-enantiomer in displacing (S)-(1251)iodocyanopindolol from 3-receptors of sarcolemma-enriched membranes from the guinea pig heart. (R)- and (S)-propafenone exert different beta-blocking actions but equal effects on the sodium channel-dependent antiarrhythmic class 1 activity. Antiarrhythmic class 1 therapy with reduction of beta-blocking side effects may be attained with optically pure (R)-propafenone hydrochloride instead of the currently used racemic mixture.