Synchronization of tumor cells can facilitate measurement of cycle time and permit identification of subpopulations which deviate from the average in either total cycle time or duration of specific phases. We utilized exposure to high concentrations of thymidine to arrest cycling cells in tumor fragments in culture. Pieces of six squamous cell carcinomas, induced in heterotopically transplanted rat tracheas by exposure to benzo(a)pyrene, were placed in culture and subjected to two sequential thymidine blocks. The labeling indices in cultures studied at 2 and 8 hr after release from the second block were equal to the growth fractions. By 16 hr after release from the block, no DNA synthesis was observed in any culture. In three tumors for which cycle and cycle phase duration was measured, mitosis occurred synchronously 12 hr after release from the thymidine block, and a second period of DNA synthesis began 16 hr later. Tc was 28 hr, Ts averaged 9 hr, Tg1 averaged N hr, and Tg2 averaged 3 hr, allowing 1 hr for TM. These numbers correspond to the values for normal cells participating in wound healing in this epithelium. There were also no deviant populations present. The cycle and its phases therefore cannot serve as markers for cancer in this tissue, but the degree of synchrony achieved will permit correlative studies of phase-specific cell properties which have been suggested as candidates for markers of cancer.