Two processes contribute to the change in hepatocyte ploidy following partial hepatectomy: one is age‐dependent, while the other depends on the time which has elapsed since hepatectomy. While in the normal liver hepatocyte DNA content increases with the age of the cell, following hepatectomy the average DNA content (or ploidy) for the entire population rises as well. Hepatocyte age was derived from the cell's distance from the portal tract. We have previously shown that hepatocytes are formed adjacent to the portal tract and stream toward the terminal hepatic vein, advancing 2 μm/day. This finding enables the estimation of hepatocyte age from its location, since the older a cell the more remote it is from the portal tract. Twenty‐four young male random‐bred rats were partially hepatectomized and killed in groups of four animals at the following times: 1 h and 1, 2, 3, 7 and 14 days. Liver sections and nuclear suspensions which were collected by fine needle aspiration were stained with Feulgen and measured with the aid of computerized image cytometry. Part of the suspension which was aspirated with a fine needle was stained with propidium iodide and measured by flow cytometry. Both nuclear area and the DNA content increase with the age of the cell. The older a cell, the higher its ploidy. Generally, young hepatocytes which synthesized DNA did also divide, while older cells tended to accumulate DNA and became polyploid. The average nuclear DNA content of the entire population increased by 14% in 3 days. Partial hepatectomy thus triggers cells to synthesize DNA and proliferate. Since not all cells which synthesize DNA divide, it is proposed to distinguish between two triggers, one initiating DNA synthesis and the other, proliferation.