Infusion of tryptamine (1-4 mug/ml) through the pulmonary circulation of rat isolated lung perfused with Krebs solution caused release of a mixture of spasmogens contracting isolated smooth muscle preparations. One component of this mixture had biological activity comparable to E-type prostaglandins. Other components included a slow reacting substance comparable to SRS-A and a rabbit aorta-contracting substance comparable to RCS. Infusions of 5-hydroxytryptamine, acetylcholine and histamine (0.5-2 mug/ml) also caused release. Release induced by the tryptamines but not that by acetylcholine and histamine was prevented by methysergide whereas acetylcholine-induced release was prevented by hyoscine which did not affect tryptamine-induced release. The tryptamines and histamine released spasmogens from dog isolated lungs but only histamine was effective in guinea-pig lungs. We conclude that amine-induced release from isolated lungs is a fairly general phenomenon and that it may represent an endocrine function of lung.