Sodium morrhuate and sodium tetradecylsulfate are injected during endoscopic scle-rotherapy to control variceal bleeding. When administered to sheep they cause transient pulmonary hypertension and increase protein poor lung lymph flow. To determine the etiology of these alterations, we studied three groups of sheep after establishing acute lung lymph fistulas. In Group 1, indomethacin or ibuprofen was infused. In Group 2, 2.5 cc of sodium morrhuate was injected alone (2A) or after indomethacin or ibuprofen pretreatment (2B.) In Group 3, 2.5 cc of sodium tetradecylsulfate was given intravenously either alone (3A) or after indomethacin or ibuprofen (3B). When sclerosing agents were given alone (Group 2A and 3A) pulmonary artery pressures increased three-fold at 30 seconds postinjection to 37 · 4.4 and 39 · 5.7 mmHg respectively with a slow return to baseline over two hours. Lymph flow increased significantly from 1.3 · 1.5 to 2.7 · 1.5 cc/30 minutes after sodium morrhuate and from 1.2 ± .62 to 2.7 · 1.7 cc/30 mins at 30 minutes after sodium tetradecylsulfate and the lymph/plasma albumin ratio fell. Increased lymph flow persisted through 120 minutes. In those animals receiving a sclerosing agent after indomethacin or ibuprofen (2B and 3B) there was no change in pulmonary artery pressure, lymph flow, lymph plasma albumin ratio, or lung wet weight to dry weight ratios. We conclude that the pulmonary hypertension and increased protein poor lymph flow are mediated by prostaglandins.