Furosemide 20 mg/kg was given intravenously to 12 anesthetized dogs with clamped renal pedicles. Thoracic duct lymph flow (TDLF) increased promptly by 38% (P less than 0.05), an increment that lasted 80 min. Because in 6 of 12 dogs there was a transient increase in splanchnic blood flow, in separate groups splanchnic blood flow was either markedly constricted or markedly increased by intravenous isoproterenol. Thoracic duct lymph flow increased by 95 and 90%, respectively, following furosemid despite no further change in splanchnic blood flow. Furosemide had no effect on blood pressure, lymph protein, or plasma sodium. In four chronic caval dogs, TDLF was increased by 400%, yet furosemide produced a further increment in lymph flow of 30% (P less than 0.05). Infusion of a 25% albumin solution to contract the interstitial fluid did not abolish the furosemide effect, but a 10% mannitol solution did. Furosemide increased TDLF even after the infusion of papaverine reduced blood pressure to 60 mmHg. We conclude that furosemide increases TDLF by acting directly on splanchnic capillaries to allow increased filtration of fluid in the absence of increased splanchnic blood flow or capillary hydrostatic pressure.