The duration of DNA synthesis of a diploid cell line of Chinese hamster fibroblasts was determined in a comparative study by the FLM technique, and also by a new technique for measuring the rate of DNA synthesis of individual cells. These methods produced comparable results when applied during exponential growth of the cells. The rate of DNA synthesis was measured by means of quantitative autoradiography following a short-term incubation of the cells with 5 X 10(-6) M FUdR and 10(-5) M 14C-TdR. The choice of the medium for this purpose did not seem to be critical. The autoradiographic silver grains over cells and 14C-standard sources are counted by microphotometry using incident light bright-field. The direct measurements of DNA synthesis rate are 'compartment' statistics which have been converted into 'flux' parameters for comparison with the FLM method and applicability in cell-kinetic calculations. Frequency distributions of the rate of DNA synthesis of individual cells thus obtained may resemble normal distributions quite closely. They result from several factors: differences in the rate of synthesis in different parts of the S-phase, the density distribution of cells within the S-phase, the variation in the time of DNA synthesis among individual cells, and the experimental error. In the case of a pronounced partial synchronization as probably has been present in one experiment performed in the lag phase, an incorrect time of DNA synthesis may result from the rate values. Due to the variation in DNA synthesis rate in different parts of the S-phase it is not possible to determine the duration of DNA synthesis of an individual cell. However, the mean values of DNA synthesis time are reliable. The new method will be preferentially applied for determining the duration of DNA synthesis of human cells in as far as difficulties are encountered with the classical methods. In addition, it may be used to advantage for studying cells which make up low percentages in mixed populations. It finally permits a safer morphological classification of the cells under study than is possible with the classical methods.