1 In an open controlled study a group of 18 healthy volunteers received either pindolol 10 mg three times daily or metoprolol 100 mg three times daily for 4 weeks. Before treatment, and after abrupt withdrawal the resting heart rate, the blood pressure, the exercise heart rate and the isoprenaline CD25 (dose of isoprenaline to increase the heart rate of 25 beats/min) were determined. Heart rates were continuously monitored by an ECG-coupled computer. The CD25 values were calculated by an off line computer procedure from the on line recorded data. 2 After metoprolol we found 6 out of 12 patients with a CD25 below baseline, in one case with a corresponding increase in heart rate during exercise. After pindolol we observed a CD25 below baseline only in one case with no corresponding reaction in the exercise test. In both groups we observed a reactive increase in resting heart rate and systolic blood pressure around day 5 after withdrawal. 3 We conclude that abrupt withdrawal of metoprolol in contrast to pindolol is associated with a higher risk of developing beta-adrenergic receptor hypersensitivity.