To investigate O2- generation in in situ lungs of rats treated with drugs to induce experimental acute respiratory distress syndrome, phorbol myristate acetate (PMA) or endotoxin were injected into rats, who had been continuously infused with 2(methyl-6-[p-methoxyphenyl]-3,7-dihydroimidazo[1,2-alpha]pyraz in-3-one (MCLA) and the chemiluminescence in in situ lung was detected by a sensitive photon counter. In PMA-treated rats, two phases of chemiluminescence over non-specific chemiluminescence were observed. The first phase luminescence was sensitive to Cu-Zn superoxide dismutase, while the second phase chemiluminescence was less sensitive to Cu-Zn superoxide dismutase. Similar chemiluminescence was detected in the rats treated with endotoxin instead of PMA, but not in neutropenic rats.