text: When DLD-1/FdUrd cells expressing increased TS mRNA were treated with FdUrd and F3(d)Thd for only 4 h, the resistance ratios of DLD-1/FdUrd cells to parental DLD-1 cells were markedly different for FdUrd and F3(d)Thd, suggesting that the cytotoxicity with short-time exposure to << F3(d)Thd >> is due to a mechanism other than [[ TS ]] inhibition, although the cytotoxicity of F3(d)Thd in the short-time is low compared to that of long-time exposure.