Atenolol 100 mg once daily and metoprolol SA 200 mg once daily were compared in 20 patients with mild to moderate hypertension, in a randomised, double-blind cross-over trial. Blood pressure and heart rate were compared at rest and during maximal exercise on a bicycle ergometer at 2 and 24 h post-dose. Blood pressure control was similar with both drugs, except at 2 h when systolic blood pressure was significantly lower with atenolol. Blood pressure and heart rate were less well controlled at 24 h than at 2 h with both drugs, but the majority of patients were still considered adequately controlled at 24 h. Atenolol and metoprolol SA can be considered equally effective in the treatment of mild to moderate hypertension and can be recommended as a once daily regime.