The effects of a 2 h exposure to 250 and 400 p.p.m. of SO2 delivered via an endotracheal tube on the reactivity of the dog lung have been studied. Forty-eight hours after exposure to 250 p.p.m. SO2 base-line values of total lung resistance and dynamic lung compliance were unchanged but there was an increase in the bronchoconstrictor response to histamine (20 micrograms/kg I.V.). This enhanced response was vagally dependent. The response of lung irritant receptors to histamine (20 micrograms/kg I.V.) was also increased in these animals. Forty-eight hours after exposure to 400 p.p.m. SO2 resting total lung resistance had increased and resting dynamic lung compliance had fallen. Vagotomy produced a small but insignificant fall in resting total lung resistance. Changes in total lung resistance produced by acetylcholine (40 micrograms/kg I.V.) histamine (20 micrograms/kg I.V.) and 5-hydroxytryptamine (20 micrograms/kg I.V.) were all increased after exposure and these increases were significantly reduced by vagotomy. The falls in dynamic lung compliance produced by these agents were not enhanced by exposure to SO2. The rise in total lung resistance by bilateral vagal stimulation was increased 48 h after exposure to 400 p.p.m. SO2. Exposing dogs to SO2 produced an increase in the reactivity of the lung principally by the enhancement of reflexes. The mechanisms involved are discussed.