The effect has been studied of sodium cromoglycate (SCG) on the activity of 'C' fibre sensory nerve endings in the canine lung. Pretreatment with SCG (100 microgram/kg i.v.) reduced the excitation of these endings by capsaicin (10 microgram/kg i.v.) for approximately 45 min. This property of SCG may explain its ability to suppress certain types of bronchoconstrictor responses in man.