Atenolol is a beta-adrenergic blocking agent that blocks the effects of adrenergic drugs, for example, adrenaline or epinephrine, on nerves of the sympathetic nervous system. Atenolol reduces the heart rate and is useful in treating abnormally rapid heart rhythms. Atenolol also reduces the force of contraction of heart muscle and lowers blood pressure. Chlorthalidone is a diuretic (water pill).