A variety of liver and hepatoma cell lines were incubated in the presence of corticosterone. DNA synthesis was inhibited in some but not all cell lines. One of them, the Fu 5a clone of the Reuber hepatoma cell line, was subjected to corticosterone for 48 h. Incorporation of labeled thymidine into DNA was inhibited up to 10%. Afterward, corticosterone was removed. DNA synthesis then increased 16‐fold relative to controls. It was concluded that corticosterone blocks these cells in late G ₁ phase of the cell cycle and thus leads to synchronization.