Nitrobenzo[a]pyrenes are found in urban air particulates and particulates from diesel exhaust, gasoline engines and wood burning stoves. Following exposure of Chinese hamster ovary cells (CHO-K1-BH4) to 3-nitrobenzo[a]pyrene (3-NB[a]P), cells with multiple nuclei and/or nuclei with multiple lobes were observed. When CHO cells were treated with 5 micrograms/ml 3-NB[a]P for 5 or 20 h, aneuploidy was noted in these cells at 24-96 h post-exposure. The addition of N6, O2'-dibutyryl adenosine 3':5'-cyclic monophosphate to 3-NB[a]P-exposed CHO cell cultures appeared to reduce the amount of aneuploidy in treated cultures. Structure--activity studies showed that 1-NB[a]P was a much less effective inducer of aneuploidy than 3-NB[a]P and 6-NB[a]P was ineffective. 1-, 3- and 6-nitrosobenzo[a]pyrenes were not effective inducers of aneuploidy in CHO cells, and aneuploidy was not observed in cultures treated with 3-NB[a]P in the presence of S9 activation. It appears that the parent 3-NB[a]P is responsible for producing aneuploidy in CHO cells.