Alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase, was used to study the effect of polyamine depletion on delayed heat sensitization in Chinese hamster ovary cells (CHO). The cells were treated with 1 or 10 mM DFMO for 8 or 48 h and then given a single heat treatment (43 degrees C, 90 min) at intervals up to 150 h after DFMO addition. Cellular survival, DNA polymerase activity, and polyamine levels were measured. Delayed heat sensitization for cell lethality began 50-55 h (about two cell divisions) after addition of 10 or 1 mM of DFMO for 8 or 48 h, respectively; i.e., cell survival of heated control cells was about 10(-1), but decreased to 10(-4)-10(-5) in heated DFMO-treated cells by 100 h. During this same interval, delayed heat sensitization also was observed for loss of DNA polymerase beta activity (from 20% in cells heated without DFMO treatment to 7% in heated DFMO-treated cells), but none was observed for DNA polymerase alpha activity. Delayed heat sensitization disappeared at 120-130 h after DFMO addition, with survival of heated DFMO-treated cells returning to that for heated control cells. The onset of delayed heat sensitization occurred 30-40 h after intracellular levels of putrescine and spermidine were depleted by more than 95%; however, spermine levels were not lowered, and in some cases even increased. Levels of putrescine and spermidine increased 5-10 h before delayed heat sensitization disappeared. While putrescine reached 25% of control, spermidine exceeded control levels during this time. Furthermore, delayed heat sensitization could be reversed by adding 10(-3) M putrescine or 5 X 10(-5) M spermidine 85-95 h after DFMO addition; in both cases spermidine increased 5-10 h before the decrease in heat sensitization. Finally, neither delayed heat sensitization nor depletion of spermidine was observed in nondividing plateau-phase cells treated with DFMO, although putrescine was depleted. These results lead to the hypothesis that DFMO-induced heat sensitization which occurs after inhibition of the synthesis of putrescine is secondary to the depletion of spermidine in some critical compartment of the cell or to a biochemical alteration. This depletion or biochemical alteration apparently occurs as the cells divide about two times after the intracellular levels of soluble spermidine have been depleted.