1. Ten healthy subjects received, on separate occasions, intravenous infusions of 0.9% NaCl solution (saline) and of sodium cromoglycate given in a double-blind cross-over fashion.

2. After 20 min of infusion the specific airway conductance (sGaw) of each subject was measured in a body plethysmograph. The subject then breathed first a low (1-20 p.p.m.) and then a higher (4-40 p.p.m.) concentration of sulphur dioxide (SO2) for 2 min while the infusion continued. Measurements of sGaw were repeated after each inhalation of SO2.

3. Sodium cromoglycate infusion did not affect the sGaw measured before challenge with SO2.

4. During the infusion of sodium cromoglycate, the falls in sGaw in response to both concentrations of SO2 were significantly less than those which occurred during saline infusion.

5. Plasma concentrations of sodium cromoglycate were found to be 209 ± 22 nmol/l at the end of the drug infusion. This is about one-hundredth of lowest dose required to stabilize mast cells in vitro.

6. We conclude that sodium cromoglycate acted by a mechanism which did not involve mast cells. Other possible modes of action are discussed.

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