The production of superoxide and hydrogen peroxide and the oxidation of NADPH by goat lung microsomes in the presence of 3MI or nitrofurantoin were investigated. Although nitrofurantoin strongly enhanced microsomal formation of adrenochrome from epinephrine (a superoxide dependent reaction), 3MI did not increase the rate of adrenochrome formation by microsomes. Neither 3MI nor nitrofurantoin stimulated significant increases in microsomal hydrogen peroxide production. 3MI tripled NADPH oxidation compared to controls, while nitrofurantoin caused a nearly twenty-fold increase in NADPH oxidation. It is concluded that production of superoxide, hydrogen peroxide and depletion of intracellular stores of reduced pyridine nucleotides are not significant components of 3MI pneumotoxicity.