OKY 046, a specific thromboxane synthase inhibitor, was used to investigate whether large pulmonary emboli, like microemboli, cause an increase in thromboxane A2 and an associated increase in vascular permeability in sheep. Nineteen sheep were anaesthetised and had cannulas inserted into the afferent lymphatic of the caudal mediastinal lymph node and pulmonary and carotid arteries. Several days later the animals were pretreated with placebo or OKY 046 0.4 mg/kg one hour before being given clotted blood 0-5 g/kg intravenously. After embolisation in the control animals mean pulmonary artery pressure (MPAP) rose from 12 to 34 mm Hg and pulmonary artery wedge pressure (PAWP) fell from 4-4 to 1.5 mm Hg; the cardiac index did not change but the physiological shunt ((sS/T) rose from 17% to 50%. One hour after embolisation the platelet count fell from 76 to 32 × 106/1 whereas at 15 minutes thromboxane B2 rose from 116 to 560 pg/ml in plasma and from 324 to 795 pg/ml in lymph (p < 0.05). By 2 hours the concentration of thromboxane B2 was higher in lymph than in plasma. Lymph flow rose from 8-7 to a maximum of 27 3 ml/h at 15 minutes but despite the increase in flow the lymph:plasma (L:P) protein ratio did not fall, indicating an increased permeability of the blood vessels to protein. Pretreatment with OKY 046 inhibited the rise in plasma and lymph thromboxane B2, and limited the rise of 4S/QT. The changes in MPAP, PAWP, cardiac index, platelet count, lymph flow, and L:P protein ratio, however, were no different from those in untreated sheep. These results indicate that a large pulmonary embolus leads to an increase in plasma and lung lymph thromboxane A2, which moderates the rise in sS/OT in part but not the increase in vascular permeability.