A method of treating mucus hypersecretion, the causative factor in chronic obstructive pulmonary disease (COPD), asthma and other clinical conditions involving COPD, comprises administering a compound that inhibits exocytosis in mucus secreting cells or neurones that control or direct mucus secretion. Also described is a compound, for use in the treatment of hypersecretion of mucus, which inhibits mucus secretion by inhibiting mucus secretion by mucus secreting cells, and/or inhibiting neurotransmitter release from neuronal cells controlling or directing mucus secretion.