It has been previously demonstrated that alpha-phenyl-N-tert-butyl-nitrone (PBN), one of the most widely used free radical trapping agents, is rapidly absorbed, evenly distributed among a wide range of tissues, and metabolized to form one metabolite. The objective of this study was to determine if the distribution and metabolism of PBN can be affected by inducers or inhibitors of the microsomal mixed function oxidase (MFO) system. Rats were pretreated with MFO inducers (phenobarbital and 3-methylcholanthrene) or MFO inhibitors (metyrapone and piperonyl butoxide) before 14C-PBN was injected (i.p.) The concentrations of 14C-PBN and its metabolite were measured in plasma, urine, liver, lung and kidney 2 hours after injection. The results indicated that when MFO was induced, the concentration of 14C-PBN metabolite was significantly reduced in all tissues measured. The maximum concentration of PBN parent compound in the tissues where MFO was induced was 50% of that found in saline controls. Manipulation of tissue MFO levels with inducers and inhibitors altered the ratio of 14C-PBN parent compound to the PBN-metabolite. When 14C-PBN was incubated with rat liver microsomes at 37 degrees C in the presence of NADPH, the rate of metabolism was 1,752 dpm of 14C-PBN-metabolite formed/nmole P-450/min. Inactivation of MFO by heat (80 degrees C for 1 min) or deletion of NADPH diminished the formation of PBN metabolite in vitro. It is concluded that the MFO system may be responsible for the metabolism of PBN. Tissue concentrations of PBN can be affected by drugs or toxins which are inducers or inhibitors of MFO.