The amounts of DNA in individual cells of synchronized or log phase cultures can be determined from flow cytometry measurements of DNA:propidium fluorescence. We have used a model based on cell transit times through the cell cycle to obtain rates of DNA synthesis corresponding to different parts of the S phase of log phase cells. The five rates predicted by this method for log phase HeLa cells correlate well with those obtained by following the progression of highly synchronized HeLa cells through S phase. The absolute values of the rates can be expressed in channel number (proportional to fluorescent intensity), picograms of DNA, number of nucleotides, or molecular weight of DNA versus time, when the total cell cycle time is known, or in relative units, when it is not. The fastest rates for HeLa and 2RA cells are at the beginning and end of S phase, whereas WI-38 cells exhibited a gradual increase in rates from beginning to end of S phase. The determination of rates of DNA synthesis in log phase cultures, rather than synchronized ones, represents a substantial reduction in time, eliminates the possibility of induced metabolic artifacts as a result of the synchronization procedure, and permits the screening of many cell types that may not be amenable to precise synchronization.