The effects of two beta-blockers, one non-selective, propranolol, the other beta 1 selective, metoprolol, were studied in 8 asthmatic patients. Practolol and a placebo were used as reference products. Heart rate, blood pressure, MEV, and muscle tremor were measured at rest and after isoprenaline infusions at different concentrations. In patients receiving placebo, isoprenaline caused an increase in heart rate, MEV, systolic BP, and muscle tremor, and a reduction in diastolic BP. After propranolol, the effects of isoprenaline on heart rate, MEV, and muscle tremor were almost completely blocked, and modifications in systolic and diastolic BP were only slightly affected. Metoprolol and practolol did not inhibit the increase in MEV induced by isoprenaline, but they reduced the increase noted in the heart rate. These results demonstrate the beta 1 selectivity of these two beta-blockers. Contrary to propranolol, metoprolol and practolol did not block the increased muscle tremor provoked by isoprenaline: muscle tremor must be dependent on beta 2-receptor activity. As the effect of isoprenaline on the MEV was not inhibited by beta 1-selective beta-blockers, these can be prescribed for asthmatic patients in association with beta 2 stimulants.