In 20 unanesthetized sheep, we measured lung lymph flow and lymph and plasma protein concentrations during steady-state base-line conditions and during steady-state elevations of pulmonary microvascular hydrostatic pressure (range 3 to 23 cm H2O). In every sheep there was a base-line lung lymph flow (average 5.7 +/- 2.5 (SD) ml/hour), demonstrating that net fluid filtration occurred. The base-line lymph-plasma total protein ratio averaged 0.69 +/- 0.05, indicating a high protein osmotic pressure in the interstitial fluid at the filtration site. Lymph flow increased and lymph protein concentration decreased approximately linearly whenever hydrostatic pressure rose. A new steady-state condition was reached in 1-2 hours. The difference in plasma-to-lymph protein osmotic pressure increased by half the hydrostatic pressure increment (50% negative feedback regulation). Extravascular lung water content, measured post-mortem, did not change significantly until microvascular hydrostatic pressure more than doubled, indicating a large safety factor that protects the lungs against fluid accumulation normally. The major contributions to the safety factor appeared to be a sensitive and efficient lymph pump coupled to a washout of interstitial protein. The fluid filtration coefficient, whose calculation required many assumptions, averaged 1.64 +/- 2.65 ml/(cm H2O times hour) in the base-line condition and did not change significantly over the pressure range studied.