The effect of an increase in microvascular hydrostatic pressure, Pmv, and vasodilator prostanoids on initial burn edema formation and that occurring five to six days postburn was determined. Burn injuries in adult sheep and lymph flow, QL, were used as an indication of edema. A 2-3 mm Hg increase in Pmvat eight hours postburn resulted in a several-fold increase in protein-rich lymph flow, whereas inhibition of local release of prostacyclin had no beneficial effect. An increase in Pmvof 3 mm Hg at five days postburn still resulted in a two-fold increase in protein-rich QL. Stimulation of local prostanoid and other vasoactive mediators released with endotoxin did not accentuate QL, whereas ibuprofen injection had no beneficial effect. It was concluded that an increase in protein permeability was still present in burn tissue microvessels at five days. The rate of edema formation was markedly accentuated both initially and at five to six days by an increase in Pmv, whereas vasodilator prostanoids and inflammatory mediators released with local sepsis had no additional effect, suggesting maximum vasodilatation.