Nedocromil sodium is a new chemical entity which shows similar properties to sodium cromoglycate (SCG) and in addition exhibits a preferential activity in stabilizing mucosal mast cells. We have compared the effect of inhalation of nebulized placebo, SCG and nedocromil sodium on the bronchoconstrictor response to inhaled adenosine monophosphate (AMP) in eight atopic asthmatic subjects aged 25 yr (range 21–32 yr). The geometric mean provocation doses of AMP required to produce a 20% decrease in FEV1 (PD20 FEV1) and a 40% decrease in V˙max30 (PD40 V˙max30) following placebo were 4.9 (0.3–14.2) and 1.8 (0.1–8.4) μmol respectively. Prior inhalation of both SCG and nedocromil sodium significantly inhibited the bronchoconstrictor response to AMP with PD20FEV1s of 36.6 (4.0–132.7) and 134 (12.4–560), and PD40 V˙max30 values of 20.5 (1.4–110) and 101.6 (5–560) μmol respectively (P<0.001). Nedocromil sodium was 3.9 (FEV1) and 8.0 (Vmax30) times more potent than SCG (P<0.001). In conclusion, both drugs inhibit the bronchoconstrictor response to inhaled AMP, and nedocromil is at least 4–8 times more potent than SCG.