The bronchodilator and cardiac effects produced by aerosols of 0.5% salbutamol and 0.5% and 1% rimiterol administered for three minutes in 40% oxygen by intermittent positive-pressure ventilation (I.P.P.V.) were compared in 15 asthmatic patients. Salbutamol and both the concentrations of rimiterol were equipotent in peak bronchodilator effect, but salbutamol had a significantly longer duration of bronchodilator action. There was significantly less increase in heart rate after rimiterol than after salbutamol. Aerosols of 0.5% rimiterol, 0.5% salbutamol, and saline were administered by I.P.P.V. to 10 normal volunteers. There was no difference between the mean heart rates after 0.5% rimiterol and saline but a highly significant increase in mean heart rate was observed after 0.5% salbutamol. It was concluded that 0.5% rimiterol was an effective short-acting bronchodilator drug with little or no cardiac beta(1)-adrenergic activity when administered for three minutes by I.P.P.V. in 40% oxygen.