Salmeterol is a new inhaled beta 2 adrenoceptor agonist, which has been shown in animal experiments to produce a more prolonged bronchodilator effect than currently available beta 2 adrenoceptor agonists. It was studied in eight adult asthmatic patients. Each patient received on separate test days salbutamol 200 micrograms and salmeterol 50, 100, and 200 micrograms according to a randomised, double blind, crossover design. FEV1, peak expiratory flow (PEF), heart rate, blood pressure, and tremor were recorded in the clinic for six hours after drug inhalation; PEF was recorded for a further six hours at home. All three doses of salmeterol produced peak increases in FEV1 (mean 0.5-0.8 l) and PEF (71-100 l/min) similar to those produced by salbutamol 200 micrograms (0.5 l and 74 l/min). After salbutamol FEV1 and PEF had returned to baseline within six hours, but after all three doses of salmeterol more than half of the maximum bronchodilator effect remained after 12 hours. The effects of salbutamol and the two lower doses of salmeterol (50 and 100 micrograms) on cardiovascular measurements and on tremor were similar, whereas after salmeterol 200 micrograms there was a small decrease in diastolic blood pressure and an increase in heart rate and tremor. Thus inhaled salmeterol has a long acting bronchodilator action in asthmatic patients. This effect may be of value in the treatment of asthma, particularly in patients with nocturnal symptoms.