Pulmonary microsomal lipid peroxidation was studied in-vitro with regard to the effects of NADPH, ferrous ion (Fe(2+)), ferric ion (Fe(3+)), and ascorbate. Lungs were dissected from male English- short-hair-guinea-pigs, and the microsomes were obtained from lung homogenates by differential centrifugation. Lipid peroxidation was determined by the production of malonaldehyde in the presence or absence of the substances used to initiate or modify peroxidation. The addition of 2.5 millimolar (mM) Fe(2+), Fe(3+), or ascorbate increased pulmonary microsomal lipid peroxidation in the increasing order of Fe(3+), ascorbate, and Fe(2+). The addition of ascorbate enhanced Fe(3+) stimulated lipid peroxidation but reduced the magnitude of the Fe(2+) stimulated lipid peroxidation. NADPH stimulated lipid peroxidation in normal pulmonary microsomes but not in heat treated preparations or preparations containing 1mM ethylenediaminetetraacetic-acid. Fe(2+) and phosphate were required for maximal microsomal stimulation by NADPH, but the addition of NADPH to the Fe(2+) medium decreased microsomal stimulation when the iron concentration was greater than 1mM. The results were compared to similar experiments using liver microsomal preparations, and the authors conclude that there are significant differences in the behavior of pulmonary and hepatic microsomes.