The effect of heat on the induction and repair of DNA single (ssb) and double (dsb) strand breaks was studied in irradiated exponentially growing or plateau-phase CHO cells and their DNA dsb repair-deficient, radiation-sensitive counterpart, the xrs-5 cells. Induction and repair of DNA ssb was measured by the alkaline unwinding technique, whereas induction and repair of DNA dsb was measured by the non-unwinding filter elution technique. The results indicated that pre-exposure of cells to heat (45 x 5 degrees C) for 8-30 min did not affect the induction of DNA ssb or DNA dsb per Gy and dalton of DNA in CHO or xrs-5 cells, tested either in the exponential or in the plateau-phase of growth. On the other hand, pre-exposure to heat inhibited DNA repair processes and increased the fraction of unrepaired radiation-induced damage measured 2 h after irradiation. Repair of DNA dsb was more heat-sensitive than repair of DNA ssb in both cell lines. Repair of radiation-induced ssb or dsb was inhibited in xrs-5 cells to a larger extent than in CHO cells after a similar exposure to heat. These results complement those previously reported on heat-induced radiosensitization in these cell lines, and suggest that the reduction in heat-induced radiosensitization observed in xrs-5 cells is largely due to their deficiency in repairing DNA dsb, rather than to a reduction in the ability of heat to inhibit DNA repair processes in general. The data presented here provide further support to the hypothesis that DNA dsb repair proficiency is a prerequisite for heat-induced radiosensitization.