Atenolol is in a group of drugs called beta-blockers. Beta-blockers affect the heart and circulation (blood flow through arteries and veins). Chlorthalidone is a thiazide diuretic (water pill) that helps prevent your body from absorbing too much salt, which can cause fluid retention. The combination of atenolol and chlorthalidone is used to treat hypertension (high blood pressure). Atenolol and chlorthalidone may also be used for purposes not listed in this medication guide.