The effects of 50, 100 and 200 mg oral single doses of metoprolol were compared in 23 patients with stable angina pectoris, previously treated with 50 mg metoprolol t.i.d. Treatment was stopped for one week and during this week patients received 50, 100 or 200 mg metoprolol in a randomized order as a single dose, with 48 hours between doses. Exercise tests were performed on a bicycle ergometer 1.5 h after tablet intake. At rest, systolic blood pressure was the same on all three doses while the heart rate was lower after 200 mg than after 50 mg of metoprolol (57 and 54 beats/min, respectively; p less than 0.01). During exercise, mean heart rate at the highest comparable work load was 101, 95 and 89 beats/min on 50, 100 and 200 mg metoprolol, respectively (p less than 0.001 for differences between doses). Corresponding values for systolic blood pressure were 177, 177 and 168 mm Hg. Total work performed and time until onset of pain were significantly higher on 200 mg than on 100 mg (p less than 0.05) or 50 mg (p less than 0.05). In a follow-up study which comprised one month's treatment with 100 mg b.d. and one month on 200 mg b.d., the majority of patients preferred treatment with metoprolol 100 mg b.d.