The effect of fibrin degradation products (FDP) on pulmonary transvascular fluid and protein exchange was examined in the sheep lung lymph fistula preparation. The pulmonary lymph was used to assess changes in pulmonary lymph flow (Qlym) (a measure of net transvascular fluid filtration rate) and the lymph/plasma protein concentration ratio (L/P) (a measure of protein seiving across the microvascular barrier). Studies were made in 3 groups: Control-Thrombin (n = 7) received 96.9 +/- 9.4 U/kg of alpha-thrombin, Control-FDP (n = 6) received infusion of FDP prepared by plasmin digestion of fibrin, and Thrombin-FDP (n = 5) received thrombin (102.0 +/- 7.5 U/kg) and then an FDP infusion was begun at 60 min after the thrombin. In the Control-Thrombin animals, Qlym increased without a change in the L/P after thrombin, indicating an increase in vascular permeability to proteins. In the Control-FDP group, infusion of FDP had no effect on Qlym and L/P. In the Thrombin-FDP group, thrombin increased Qlym with no change in the L/P and the subsequent infusion of FDP further increased Qlym but slightly decreased the L/P, an effect not seen in the Control-FDR group. The results indicate that FDP infusion alone does not alter pulmonary transvascular fluid and protein exchange. However, in the presence of thrombin-induced pulmonary microembolization FDP infusion enhances the net transvascular fluid filtration rate, probably by increasing the capillary hydrostatic pressure.