Atenolol and chlorthalidone is a combination medication used to treat high blood pressure. Atenolol belongs to a group of drugs called beta-blockers which work by blocking the action of certain natural chemicals in the body, such as adrenaline, on the heart and blood vessels, thereby reducing heart rate and lowering blood pressure. Chlorthalidone belongs to a group of drugs called diuretics or "water pills" which help the body to get rid of excess salt and water by increasing urine output, resulting in a decrease in blood volume and lowering of blood pressure. The combination of these two drugs can help to control blood pressure more effectively than either drug alone.