Summary Some aspects of the population dynamics before, during and after the double thymidine treatment synchronization procedure were analysed. The mitotic index in the synchronized populations rose from 0.0 per cent at 1 hr after the end of the second treatment to 20.8 per cent at 9.7 hr, dropped to 0.1 per cent at about 22 hr and increased again to a second peak of about 8 per cent at 31 hr. The percentage of mitoses in the controls was 2.1. The degree of synchronization of the first generation, determined according to Engelberg, was 73 per cent. The percentage of S phase cells fell from 96 per cent at 0–3.5 hr via 50 per cent at 6.5 hr to 1 per cent at 12–15 hr, rose to 72 per cent at 24 hr and decreased again to 7 per cent at 34 hr. The mean length of the G 1 and the G 2 phases of the synchronized cycle was shorter than in the asynchronous controls, while the duration of mitosis and the S phase was about equal in both cases. Some other characteristics of the synchronized generation cycle are discussed. It was argued that the synchronization in this system probably was a consequence of a reversible alteration of the life cycle of the cells, induced by thymidine. The mean duration of the S phase was greatly increased in the presence of thymidine. Some biochemical implications concerning the blocking and deblocking of the DNA synthesis by the administration and removal of thymidine are discussed.