An accumulation of cells in early S phase was observed in normal mouse duodenal epithelium studied with flow cytometry (Cheng and Bjerknes, 1982). To determine if this accumulation of cells was the result of a lower rate of DNA synthesis, animals were given a single injection of 3H-thymidine and the epithelium collected one hour later. The epithelium was processed for flow cytometry. Seven sort windows were established in different portions of the DNA histogram. Cells from each window were sorted onto glass slides that were then processed for radioautography. The number of silver grains over the nuclei of each sorted population was counted. It was found that cells in early S phase had significantly fewer grains over their nuclei than did mid- or late-S phase cells. We conclude that the accumulation of cells in early S phase is due, at least in part, to a lower rate of DNA synthesis in early than in mid or late S phase.