The arachidonate cascade is important for the generation of reactive species (RS), and cyclooxygenase (COX) is a key enzyme of this cascade. Tissues of 24-month-old rat lung showed a 2-fold increase in RS, malondialdehyde and thromboxane B-2 than those of 6-month-old rat. We found that the effects of 50 muM H2O2 and 200 muM t-butylhydroperoxide (t-BHP) specify on COX activity, and that their effects increased cytosolic COX activity in a concentration-dependent manner (1-50 muM) in 24-month-old rat. Our results suggested that COX activators such as t-BHP and H2O2, which are located in cytosol, are essential for the activation of COX in aged lung.