TWO FIQURESI n 1952, an experiment was started concerned with the effects of x-radiation upon cell (division on regenerating rat livers. Groups of rats were partially hepatectomized and then irradiated a certain number of hours after the operations. Each group was sacrificed at 12-hour intervals following irradiation and the livers subsequently sectioned, stained, and examined. The average number of mitoses per unit volume of liver parenchyma, as compared with that of the non-irradiated controls, was used as the criterion of radiation effect. Initially, the partial hepatectomies, and, therefore, the radiation, were performed a t different hours of the day, so arranged that killing and autopsy were always carried out in the morning. To test the reliability of this procedure, two groups of animals, to be sacriiiced at the same time after radiation, were hepatectomized at reciprocal hours of the day -one group at 9 : 00-10 : 00 A.M., the other at 9 : 00-10 : 00 P.M.Except for the time difference, the general schedule followed was identical for both groups. It was noted at once that there was a wide gulf f;eparating the mean values recorded for each group. These results suggested the possibility that a diurnal periodicity in mitotic activity was responsible for thc discrepancy, and the experiment was redesigned so that all operations were carried out at the same hours, all time being reckoned thereafter from this Rhode Island.