To investigate the origin of sister chromatid exchanges (SCEs) induced by inhibitors of DNA synthesis, V79/AP4 Chinese hamster cells were treated with aphidicolin, 1-beta-D-arabinofuranosylcytosine, and thymidine. At the end of the treatments we determined both the distribution of the cells in the various phases of the cell cycle and the induction of SCEs. Our data indicate that the cells that were replicating their DNA were arrested at various stages of the S phase. By analyzing the patterns of SCE distribution, we found that the metaphases of the treated cells exhibited either "normal" or enhanced levels of SCEs. Our results suggest that the inhibitors of DNA synthesis induce SCEs in the cells in early S phase probably by activation of potential replicative origins.