The present investigation was attempted to study the abnormal DNA replication in synchronized CHO-K1 cells exposed to UV-light, Results obtained from this study are as follows: S phase of CHO-K1 cells was found to be multiphasic, with a distinct pattern in the rate of DNA synthesis. Each phase showed a characteristic response to UV-light; early replication was enhanced, while late replication was inhibited. In asynchronous populations, however, the rate of DNA synthesis was greatly reduced at the beginning but gradually recovered to control level thereafter (data not shown). During the early replication, the increased synthesis of nascent DNA molecules by UV-light resulted in the accumulation of DNA molecules less than the average replicon size equivalent to 4.03×107 daltons. The nucleoid affected by UV-light was sedimented faster than that of control, suggesting that it has a more compact structure. As S phase progressed, the remaining fractions of nascent DNA on nuclear matrix were progressively reduced, indicating that relatively late replication be initiated at the site distinct from nuclear matrix. After UV-irradiation, however, DNA replication seems to be reinitiated aberrantly on nuclear matrix.