We tested the effects of alpha-naphthylthiourea (ANTU) on lung fluid balance and prostanoid concentrations in anesthetized sheep acutely prepared for collection of pulmonary lymph. Sheep were given 20, 50, 75, or 100 mg/kg ANTU or the vehicle dimethylsulfoxide (DMSO) intravenously. The first phase of the response consisted of transient increases in pulmonary artery pressure and plasma and lymph thromboxane B2 concentrations. Lymph flow increased with no change in the lymph-to-plasma protein concentration ratio (L/P). These changes occurred in sheep given DMSO alone or DMSO with ANTU; they were not dependent on the dose of ANTU given. Two to 4 h after drug administration, pulmonary artery pressure and thromboxane concentrations were normal or near normal. Lymph flow rate reached steady-state levels averaging 1.5 (DMSO alone) to 5.3 (100 mg/kg ANTU) times base line with L/P ratios unchanged from base line. ANTU/DMSO produces transient, severe pulmonary hypertension that may be prostaglandin mediated. The sustained response consists of increased flow rate of protein-rich lymph.