1. The metabolism of some vasoactive hormones perfused through pulmonary vessels was studied in samples of human lung obtained at operations.
2. 5-Hydroxytryptamine was inactivated and this inactivation was inhibited by desmethylimipramine and mebanazine.
3. β[14C]Phenylethylamine was also metabolized in human lung.
4. Prostaglandin E2 was inactivated and this inactivation inhibited by bromocresol green and frusemide.
5. About 10% of the radioactivity from infused [14C]arachidonic acid emerged from the lung and a small amount of biological activation occurred.
6. Of [14C]arachidonic acid retained by the lung, most was present in phospholipid with lesser amounts in neutral lipid and free acid fractions.
7. The fate of the hormones studied was qualitatively similar to their fate in animal lungs.