The effect of exogenous surfactant on the pharmacokinetics of intratracheally administered recombinant human superoxide dismutase (rhSOD) was studied. Five groups of rats received the following intratracheally: 1 mL/kg of saline; 5 or 25 mg/kg of rhSOD; or 4 mL/kg of exogenous surfactant followed in 30 min by 5 or 25 mg/ kg of rhSOD. Animals were killed at 24,48, and 72 h, and serum, bronchoalveolar lavage, and lung tissue were analyzed for rhSOD. rhSOD was not detected in the lungs of saline-treated animals or in serum from any animal. At 24 h, lung-tissue rhSOD was higher in rats treated with surfactant and rhSODversusrhSOD alone (5 mg/kg: 6.8 ± 2.5versus0 μg/whole lung,p< 0.05; 25 mg/kg: 29.9 ± 9.6versus0.1 ± 0.1 μg/whole lung,p< 0.05). Bronchoalveolar lavage fluid levels correlated well with lung tissue concentrations. By 48 h, lung tissue rhSOD concentrations were insignificant in all groups. rhSOD was still present in lavage fluid from rats treated with surfactant and rhSOD. No rhSOD was detected at 72 h. In separatein vitroexperiments, physical and biological drug-drug interaction studies were performed. When radiolabeled rhSOD was combined with exogenous surfactant and centrifuged at 10000 ×gfor 30 min, 81.3 ± 2.5% of rhSOD was found in the supernatantversus18.7 ± 2.5% in the surfactant pellet. Serial washing of the surfactant pellet removed virtually all remaining rhSOD. This finding suggests that the rhSOD and surfactant were only weakly associated. Combining rhSOD and exogenous surfactant did not alter the activity of either agent. Data suggest that exogenous surfactant prolongs the t½ of rhSOD in the lung. This finding may be important in determining future rhSOD administration strategies in preterm infants with respiratory distress syndrome who receive exogenous surfactant replacement therapy.