A morphologic study of DNA synthesis and cellular proliferation in regenerating rat liver has been made by means of incorporation of thymidine-H³ and autoradiography. Appreciable numbers of hepatocytes began to synthesize DNA between 12 and 18 hours after partial hepatectomy, the number increasing rapidly until a peak value of 29.4 ± 6.2 per cent was reached at 20 hours after hepatectomy, thereafter decreasing more slowly toward normal. The peak incidence of hepatocytes synthesizing DNA preceded the peak incidence of such cells in mitosis by about 6 hours. At equivalent periods of time 6–8 times as many hepatocytes were labeled as were in mitosis. At times when near maximal numbers of hepatocytes were synthesizing DNA, such active cells were predominantly located in zones 1 and 2 of the hepatic acinus. Some cells originally labeled in these areas at 20 hours after hepatectomy were apparently forced into zone 3 by further cellular proliferation. From 0 to 72 hours after hepatectomy about 80 per cent of all new hepatocytes were formed in zones 1 and 2. Synthesis of DNA in littoral and ductal cells began 8–12 hours later than it did in hepatocytes, reaching a peak between 36 and 48 hours after hepatectomy. During the entire period of regeneration studied, about 75 per cent of total DNA synthesis could be accounted for by hepatocytes, and hepatocytes in zones 1 and 2 accounted for about 80 per cent of this.