The efficacy of oral atenolol in increasing the capacity and duration of exercise in 43 patients in sinus rhythm with mitral stenosis was evaluated and compared with that of oral verapamil in an open-label cross-over design. It was observed that although oral atenolol (100 mg per day) caused significant reductions in heart rate while resting and during exercise (P less than 0.001), the increases in capacity and duration of exercise were not significant. Oral verapamil (80 mg three times per day) also caused significant reductions in the heart rates at rest and during exercise (P less than 0.05) but the increases observed, although greater than that with atenolol, failed to reach the level of statistical significance. Occasional side effects occurred with both the drugs. Subjective symptoms of dyspnoea at rest and on exertion were relieved with both the drugs. We conclude that, although both drugs reduce the symptoms of dyspnoea, they cause only minor increases in the objective parameters. They do not, therefore, provide an alternative to surgery and have only a temporary place in the management of patients in sinus rhythm with mitral stenosis who are awaiting surgery.