We studied whether changes in lung function after burns (1- to 12-h period) were due to changes in lung water or airways resistance and the relationship of the changes to prostanoid and O2 radical activity (measured as lipid peroxidation). Twenty-five anesthetized mechanically ventilated adult sheep were given a 40% of body surface scald burn and resuscitated to restore and maintain base-line filling pressures. Dynamic lung compliance (Cdyn) decreased by 40% from 38 +/- 5 to 24 +/- 4 ml/cmH2O at 12 h. Venous thromboxane B2 transiently increased from 210 +/- 40 to 1,100 +/- 210 pg/ml, and the value in lung lymph increased from 180 +/- 80 to 520 +/- 80 pg/ml. Prostacyclin levels in lung lymph and plasma remained at base line. Protein-poor lung lymph flow increased two- to threefold, but postmortem lung analysis revealed no increase in lung water from the control of 3.5 +/- 0.3 g H2O/g dry wt. No increase in protein permeability was seen. However, the lipid peroxidation of lung tissue measured as malondialdehyde was significantly increased from the control value of 56 +/- 4 nmol/g lung to a value of 69 +/- 6. Ibuprofen pretreatment (12.5 mg/kg) markedly attenuated the decrease in Cdyn, with the value at 12 h being 90% of base line. Ibuprofen also decreased the amount of lung lipid peroxidation but did not decrease the lung lymph response. We conclude that the decrease in Cdyn seen early postburn is not due to increased lung water, but, rather, is due to a mediator-induced bronchoconstriction, attenuated by ibuprofen; the mediator being either thromboxane or a byproduct of O2 radicals as evidenced by increased lipid peroxide production in lung tissue.