The initiation of DNA replication and the subsequent chain elongation were studied using Chinese hamster ovary cells synchronized at the beginning of S phase. The cells were synchronized by a combination of mitotic selection and treatment with 5-fluorodeoxyuridine (FdU). The use of this drug at a concentration of 10(-5) M was found to effectively prevent the leakage of cells into S phase. Reversal of the FdU block by supplying thymidine resulted in the synchronous onset of initiation at multiple sites in each cell. The length of the nascent chains, as determined by autoradiography and velocity sedimentation in alkaline gradients, increased linearly with time during the first twenty minutes of S phase after release. We applied these procedures to study the effects of the length of an FdU block on the number of functional origins per cell, the rate of chain growth, and the rate of DNA synthesis per cell following reversal of the block. Although no change was noted in the rate of DNA synthesis in cells held at the beginning of S phase from 10.5 to 24 h after division, the rate of chain growth decreased from 0.94 to 0.28 microns per min. This decrease indicated that the number of functional origins increased markedly with length of FdU block. The calculated number of utilized origins per cell increased from 1,900 to 5,700. We also presented arguments that 1,900 origins per cell represents the approximate number of origins utilized by any cell held at the beginning of S phase for less than 10.5 h after division.