The longer-acting beta-receptor agonists salmeterol and formoterol are effective bronchodilators for at least 12 hours and this should be clinically useful, particularly for nocturnal asthma. Formoterol has a more rapid onset than salmeterol. There are limited dose response data on the two drugs in man but the evidence so far suggests that both drugs have roughly similar beta 2-selectivity to salbutamol and that both are about ten times as potent as salbutamol. Salmeterol may therefore have been marketed at a relatively high dose compared to salbutamol. There is no good clinical evidence to suggest that the drugs have effects other than would be expected from a beta 2-agonist with a prolonged duration of action. Medium-term studies have shown that benefit was maintained in comparison to salbutamol and, in one instance, with salmeterol when compared to placebo. Studies to date have not found any reduction in the bronchodilator response to salbutamol following regular treatment with salmeterol or formoterol, though one study has found reduced protection by salmeterol against methacholine challenge after one and two months' treatment. Longer-term safety has not been assessed.