Bronchoconstrictions of equal magnitudes were induced by 5-HT when given directly by the pulmonary artery or when inhaled as aerosol. The increase artery pressure was greater when 5-HT was infused directly into the pulmonary artery. Intravenous application of atropine was always found to be effective in blocking the bronchospasm, but not the pulmonary vasoconstrictive action. It has been concluded that 5-HT induced bronchoconstriction reflexly, whereas the mode of 5-HT action on the pulmonary vascular system is not due to a reflex involving vagal pathways. The cause of the different blood gas changes are also discussed