The bronchodilating efficacy and the degree of beta2-selectivity of rimiterol, salbutamol and isoprenaline were determined in seven asthmatic patients. Rimiterol, 0.5 (high dose) and 0.05 mug/kg/min (low dose), salbutamol, 0.3 and 0.03 mug/kg/min, isoprenaline, 0.05 and 0.005 mug/kg/min, and placebo were administered by a single intravenous injection over 6 minutes in a double-blind trial. Airway resistance, heart rate, blood pressure and skeletal muscle tremor were measured before and at various times for 2 hours after each injection. The high doses of rimiterol (37%), salbutamol (37%) and isoprenaline (32%) produced immediate and effective bronchodilatation. The duration of action of rimiterol and isoprenaline was similar and shorter than that of salbutamol. For these ventilatory responses there were heart rate increases of 32, 20 and 40 beats/min for rimiterol, salbutamol and isoprenaline, respectively. The three drugs produced similar increases in pulse pressure and tremor. Dose-responses were obtained for each drug with all parameters measured and significant differences at various times found. Isoprenaline was approximately 8 and 5 times as potent as rimiterol and salbutamol, respectively, in bronchodilator action, when equimolar doses were compared. Similarly, isoprenaline was approximately 16 and 12 times as potent in increasing the heart rate as rimiterol and salbutamol, respectively. For an equal bronchodilator action, isoprenaline increased the heart rate 2 and 2.5 times more than rimiterol and salbutamol, respectively. Rimiterol is an effective, short-acting bronchodilator, with similar beta2-selectivity to salbutamol, when administered intravenously to asthmatic patients. The relative potencies and degrees of beta2-selectivity of these drugs depend partly on their route of administration.