Mustard gas exposure causes inflammatory lung diseases. Many inflammatory lung diseases are associated with oxidative stress. Reactive oxygen species (ROS) are involved in the maintenance of physiological functions. In tissues, it is therefore essential to maintain a steady-state level of antioxidant activity to allow both for the physiological functions of ROS to proceed and at the same time preventing tissue damage. We have recently reported that mustard gas exposure decreases the overall activity of superoxide dismutase (SOD). In the present study, we investigated the effects of half sulfur mustard gas, 2-chloroethyl ethyl sulfide (CEES) on each of the three isoforms of SOD [SOD-1 (CuZn), SOD-2 (Mn) and SOD-3 (extracellular)]. Adult guinea pigs were intratracheally injected single doses of CEES (2mg/kg body weight) in ethanol. Control animals were injected with vehicle in the same way. The animals were sacrificed after 7 days and lungs were removed after perfusion with physiological saline. Lung injury was established by measuring the leakage of iodinated-BSA into lung tissue. CEES exposure caused a significant increase in the activity of SOD-1 (35%). However, the SOD-3 activity which is the predominant type in lung was significantly decreased (62%), whereas, no change was observed in SOD-2 activity. Thus the decrease in the total activity of SOD was primarily due to the SOD-3 isozyme. Northern blot analysis indicated 3.5-fold increased expression of SOD-1 in CEES-exposed lung, but no significant change in the expression of SOD-2 and SOD-3 was observed. CEES exposure did not cause mutation in the coding region of SOD-1 gene while causing modulation in expression levels. The protein levels of SOD-1, SOD-2 and SOD-3 were not altered significantly in the CEES-exposed lung. This decrease in the activity of SOD-3 may be due to the cleavage of active form of the protein to an inactive form. Existence of active and inactive forms of SOD-3 as a result of shifts in Cys-Cys disulfide bonding has been described in human, recently. Our results indicate that the overall decrease in the activity of SOD by CEES exposure is probably mediated by direct inactiviation of the SOD-3 gene or the enzyme itself. Studies are underway in our lab to investigate whether mustard gas-induced inactivation of SOD-3 lung is similarly mediated by a change in Cys-Cys disulfide bonding.