The optimal timing of administration of free radical scavengers in lung preservation has yet to be determined. In previous studies, the oxygen free radical scavenger dimethylthiourea (DMTU) has improved pulmonary function when added to Eurocollins flush solution at the time of lung harvest and infused at the time of lung reimplantation. To determine when DMTU must be administered in order to preserve lung function most effectively, 16 dogs underwent lung allotransplantation. Donor lungs were flushed with modified Eurocollins solution (50 ml/kg) and stored for 12 hr at 4 degrees C. DMTU was either added to the flush solution (5 g) or infused (20 g over 2 hr) at the time of reimplantation. Investigators were blinded to the time of DMTU administration. The contralateral pulmonary artery was ligated after a 1-hr stabilization period. Measurements were recorded for 8 hr while keeping FiO2 constant at 40%. The flush group demonstrated significantly improved survival and oxygenation compared with the infusion group. The mortality rate was 50% in the infusion treatment group, while no deaths occurred in the flush animals. Lung function is adequately preserved after 12-hr storage when DMTU is administered in the flush solution at the time of harvest. It would appear that oxygen free radical scavengers must be present in lung tissue during storage in order to be effective.