A circadian dependent delay in the incorporation of [ ³H]TdR into DNA, presumably due to variations in the intracellular pool of [ ³H]TdR derivatives, was found. It seems reasonable to relate this effect to a circadianally varying age distribution of cells in S phase. At any given time the S phase cells showed large variations in DNA synthesis rate, but it was still possible to identify a mean diurnal variation in the DNA synthesis rate. Differences in the ability of S phase cells to incorporate [ ³H]TdR are also discussed in relation to flow cytometrical measurements, and this contributes to the understanding of the commonly observed phenomenon that flow cytometry estimates of S-fractions are higher than those obtained with autoradiography.