Exposure to air pollution particles can be associated with increased human morbidity and mortality. The mechanism(s) of lung injury remains unknown. We tested the hypothesis that lung exposure to oil fly ash (an emission source air pollution particle) causes in vivo free radical production. Electron spin resonance (ESR) in conjunction with the spin trap alpha-(4-pyridyl 1-oxide)-N-tert-butylnitrone (4-POBN) was used to detect radical adducts. Rats were instilled with 500 micrograms of either oil fly ash or saline. Twenty-four hours later, ESR spectroscopy of the chloroform extract from lungs of animals exposed to the oil fly ash gave a spectrum consistent with a carbon-centered radical adduct (hyperfine coupling constants alpha N = 15.0 G and alpha H beta = 2.5 G), while those spectra from lungs instilled with saline revealed a much weaker signal. This signal was reproduced by instilling animals with the soluble fraction of the oil fly ash, which contains soluble metal compounds. The same signal was observed after instillation of either a mixture of vanadium, nickel, and iron sulfates or VOSO4 alone. We conclude that, after instillation of an air pollution particle in the rat, ESR analysis of lung tissue demonstrates in vivo free radical production. This generation of free radicals appears to be associated with soluble metals in the oil fly ash.