DNA polarity at sister chromatid exchange (SCE) sites were studied in ring chromosomes from Chinese hamster cells. If the polarity of DNA strands was conserved. a double-length, symmetric dicentric ring chromosome with symmetric twin SCEs appeared after two cell cycles when a SCE occurred in S1. Indeed, approximately two-thirds of double-length. symmetric ring chromosomes belonged to this class. One-third of them, however, did not show symmetric SCEs, suggesting that polarity was not always conserved. SCE counts at centromeric regions were not high enough to account the frequency (about 1/3) of apparently inverted rejoining sites. The hypothesis that the polarity of rejoining sites is either conserved or inverted and that illegitimate rejoinings are partially repaired could explain the results. Telomere-like structures or intermediate structures during double-strand repair processes may contribute to the inverted polarity.