Circadian variations in DNA synthesis after tritiated thymidine and autoradiography, mitotic activity with the Colcemid method, and cell size, represented inversely by the number of cells per microscopic field, are reported for the hepatocyte population of the young immature growing liver of the male mouse. The peak of DNA synthesis is at 04:00 hours and that of mitotic activity at 12:00 hours. The 8-hour interval between the two peaks is considered, in agreement with other authors, a good estimation of the lag period between DNA synthesis and the initiation of mitotic activity.The minimum cell number per microscopic field, which is considered to correspond to the maximum cell size, coincides with the starting of the circadian mitotic wave. The significance of this coincidence and the relationships of the circadian curves of the parameters controlled with the feeding pattern and food consummatory curve of standardized mice, are discussed.