Brief (less than or equal to 20 nsec) 5-15 mJ pulses of near-ultraviolet laser light were employed to promote, in a controlled manner, the reaction between 8-methoxypsoralen and the DNA of Chinese hamster ovary (CHO) cells. According to Johnston et al. [21], a single brief laser pulse should induce only monoadducts between 8 methoxypsoralen and DNA, while multiple laser pulses are needed to induce 8-methoxypsoralen--DNA crosslinks. In the present study, a single laser pulse was found sufficient to cause a significant elevation in the sister-chromatid exchange (SCE) frequency in CHO cells exposed to 0.75-1.2 X 10(-5) M 8-methoxypsoralen. Additional laser pulses caused less than proportional further increments in SCEs, although the total yield of 8-methoxypsoralen adducts in similarly exposed solutions of isolated DNA increased linearly. Higher numbers of laser pulses induced detectable amounts of 8-methoxypsoralen--DNA crosslinks and distorted the DNA flow histogram of CHO cells in a manner consistent with retarded cell cycle traverse. It is concluded that 8-methoxypsoralen--DNA monoadducts are sufficient to induce SCEs in CHO cells, while 8-methoxypsoralen--DNA crosslinks might have relatively little overall effect on SCE frequencies. These results, together with previous data correlating SCE induction and mutagenesis [10] are discussed in the context of strategies for evaluating psoralen derivatives for their suitability in the phototherapy of skin disorders.