Cells were exposed to BrdUrd for intervals of between 0 and 3 h, stained with a mixture of 33258 Hoechst and propidium iodide (PI) and analysed in a two-parametrical flow cytometer. The BrdUrd-quenched Hoechst fluorescence gives information about where the cells were located at time 0 of the incubation, and the DNA-specific PI fluorescence gives information about the actual location of the cells in the cycle. Narrow windows in the Hoechst fluorescence were selected. The location of the corresponding cells in the S phase, as determined from the peaks of the projection on the PI-axis, yielded information about how far the cells had travelled through S phase during BrdUrd incubation. Thereby, the DNA synthesis rate in subcompartments of S phase was determined and showed maximum values at approximately mid-S. In addition, information is obtained about the whole duration of the S phase and the spread of transit velocities through subcompartments of S.